Structure and Function of Chihuahuan Desert Ecosystem

• <UL> Chapter 1: Introduction Chapter 2: Regional Setting of the Jornada Basin Chapter 3: Climate and Climatological Variations in the Jornada Basin Chapter 4: Soil Development in the Jornada Basin Chapter 5: Patterns and Controls of Soil Water in the Jornada Basin Chapter 6: Nutrient Cycling within an Arid Ecosystem Chapter 7: Biogeochemical Fluxes across Piedmont Slopes of the Jornada Basin Chapter 8: Water and Energy Balances within the Jornada Basin Chapter 9: Eolian Processes on the Jornada Basin Chapter 10: Plant Communities in the Jornada Basin: The Dynamic Landscape Chapter 11: Patterns of Net Primary Production in Chihuahuan Desert Ecosystems Chapter 12: Chihuahuan Desert Fauna: Effects on Ecosystem Properties and Processes Chapter 13: Grazing Livestock Management in an Arid Ecosystem Chapter 14: Remediation Research in the Jornada Basin: Past and Future Chapter 15: Applications of Remotely Sensed Data from the Jornada Basin Chapter 16: Modeling the Unique Attributes of Arid Ecosystems: Lessons from the Jornada Basin Chapter 17: A Holistic View of an Arid Ecosystem: A Synthesis of Research and Its Applications Chapter 18: Future Directions in Jornada Research: Applying an Interactive Landscape Model to Solve Problems </UL>


• Preface


• Jornada Basin in southern New Mexico


• Jornada Experimental Range, LTER


• Research in this basin formally began in 1912


• creation in 1927 of the Chi-huahuan Desert Rangeland Research Center (CDRRC, adjacent to the Jornada Range


• Agri-cultural Research Service which assumed control of the Jornada Experimental Range in 1954


• establishment of a National Science Foundation Long-TermEcological Re-search (LTER) site in the Jornada Basin of south-central New Mexico in 1981


• Jornada Basin: http://usda-ars.nmsu.edu and http://jornada-www.nmsu.edu


• 01 Introduction


• recognizes critical points, or thresholds, in system dynamics, yet these points may be manageable for increasing system resilience.


• Table 1-1. The nine assertions of the Dahlem Desertification Paradigm and some of their implications (Reynolds et al. 2003). These assertions are not all-encompassing but provide the framework for a new paradigm.


• Assertion 1. Desertification always involves human and environmental drivers *Always expect to include both socioeconomic and biophys-ical variables in any monitoring or intervention scheme


• Assertion 2. "Slow" variables are critical determinants of system dy-namics *Identify and manage forth small set of "slow" variables that drive the "fast" ecological goods and services that matter at any given scale


• Assertion 3. Thresholds are crucial and may change over time *Identify thresholds in the change variables at which there are significant increases in the costs of recovery and quan-tify these costs, seeking ways to manage the thresholds to increase resilience


• Assertion 4. The costs of interven-tion rise nonlinearly with increasing degradation *Intervene early where possible, and invest to reduce the transaction costs of increasing scales of intervention


• Assertion 5. Desertification is a re-gionally emergent property of local degradation *Take care to define precisely the spatial and temporal ex-tent of and processes resulting in any given measure of lo-cal degradation. But don't try to probe desertification be-yond a measure of generalized impact at higher scales


• Assertion 6. Coupled human envi-ronment system change over time *Understand and manage the circumstances in which the hu-man and environmental subsystems become decoupled


• Assertion 7. The development of ap-propriate local environmental knowl-edge (LEK) must be accelerated *Create better partnerships between LEK and conventional scientific research, employing good experimental design, ef-fective adaptive feedback and monitoring


• Assertion 8. Systems are hierarchi-cally nested (manage the hierarchy!) *Recognize and manage the fact that changes at one level affect others; create flexible but linked institutions across the hierarchical levels, and ensure processes are managed through scale-matched institutions


• Assertion 9. Limited suite of pro-cesses and variables at any scale makes the problem tractable *Analyze the types of syndromes at different scales, and seek the investment levers that will best control their effects - Awareness and regulation where the drivers are natural, changed policy and institutions where the drivers are social


• Early Ranching in the Jornada Basin 01


• livestock introduced early part of the sixteenth century (Hastings and Turner 1965


• for over 250 years grazing was limited to the Rio Grande Valley


• Jornada Plain began to be settled following passage of the Homestead Act of 1862 and the end of the American Civil War in 1865. The first well on the plain was dug in 1867


• 1888 that the Detroit and Rio Grande Livestock Company pumped water from the river to a tank on the mesa and piped it to troughs 10 km inside the Jornada Basin so that cattle (Bos taurus)could graze these upland grasslands.


• owned by former U.S. Army cavalry officers from Michigan, the Detroit Company began to assemble grazing rights across the Jornada Plain during this period [20k cows]


• [land was overstocked and stressed even in good years, during drought it exceeded carrying capacity]


• Concurrent with expansion of livestock grazing in the late 1800s throughout the Southwest region was a noticeable decline in rangeland conditions (Smith 1899). In 1908, E. O. Wooton documented deteriorated rangeland conditions in New Mexico.


• Our Setting 01


• Chihuahuan Desert is the largest desert in North America


• Jornada Basin are representative of the northern Chihuahuan Desert (the Trans-Pecos region) and the Mexican Highland region of the Basin and Range Physiographic Province.


• The Chihuahuan Desert dates to about 9,000 years ago. It has been hypothe-sized that during the past 3,000 years there have been three transitions from grasslands to shrublands, each followed by a return of grasslands in southern New Mexico (Van Devender 1995).


• apparently this region has not been subject to high levels of herbivory by bison (Bison) and other native ungulates for the past 10,000 years (Mack and Thompson 1982; Bock and Bock 1993).


• Jornada Basin receives an average of 245 mm/year of precipitation, about half in monsoonal storms that derive from the Gulf of Mexico during the late summer, and the remainder in synoptic weather systems stemming from the Pa-cific Ocean during the winter months.


• Measured evaporation is about 220 cm/year


• Jornada Basin, soils are largely derived from alluvial deposits from the mountains, as well as from floodplain deposits laid down by an ancient wa-tercourse of the Rio Grande through the Jornada Valley


• Grassland sites of low resistance to grazing disturbance have shifted to alternate, stable states in which shrubs dominate (Bes-telmeyer et al. 2003a).


• population of Dona Ana County (which includes much of the Jor-nada Basin) grew by nearly 81% in the decades of the 1980s and 1990s as pop-ulation density went from 65 people per km^2 to nearly 120 people per km^2 (U.S. Census Bureau 2000).


• Similar high rates of population growth are found throughout much of the Chihuahuan Desert of Mexico and across the Southwestern United States.


• region is increasingly traversed by roads, power lines, and aqueducts, and construction activities leave barren soils subject to wind erosion, reroute and linearize natural drainage ways, and replace native arid land vegetation with more profligate users of water. This desert area, like others, is in flux as the various pressures to increase economic production to support a growing human population


• Prior Research Themes 01


• community ecology, rangeland management, animal husbandry, rangeland improvement, ecosystem sciences, and interdiscipli-nary studies (Havstad and Schlesinger 1996).


• little formal, mechanistic understanding of what might have caused the complete re-configuration of vegetation and soil resources on the Jornada landscape. It was possible that numerous factors, including fire suppression, rising concentrations of atmospheric carbon dioxide, and changes in the seasonal distribution of rainfall, had contributed to large changes


• quite possible that shrub encroachment was a result of a series of events occurring over centuries, and not a response to recent livestock overgrazing (Fredrickson et al. 2005).


• As in all LTER sites, the LTER studies in the Jornada Basin were initially organized into the following five core areas (Callahan 1984): <OL> Pattern and control of primary production. Spatial and temporal distribution of populations selected to represent trophic structure. Pattern and control of organic matter accumulation in surface layers and sediments. Patterns of inorganic input and movements through soils, ground water, and surface water. Pattern and frequency of disturbance to the research site. </OL>


• development of remediation technologies for degraded landscapes requires a thor-ough understanding of the processes associated with disturbance


• above-ground net primary production (ANPP)


• sampling shrublands at a scale of 10- to 100-cm intervals, we found enor-mous variation in the content of nitrogen among soil samples. When we sampled grasslands at the same spatial scale, the soil samples seemed rather homogeneous in basic soil characteristics (Schlesinger et al. 1990, 1996). Of course, ecologists have long recognized patches, or "islands", of fertility from under shrub vegeta-tion, which leads to a heterogeneous distribution of soil resources in deserts.


• we hypothesized that desertification of semiarid grasslands may not be so much associated with a change in vegetation production as with an increase in the spatial heterogeneity of soil resources (Schlesinger et al. 1990) (see figure 1-3)


• Loss of vegetation raises regional albedo as well as regional air tem-peratures (Bryant et al. 1990). Barren soils are a source of windborne dust, which can affect the radiative balance of the planet, depending on the mineralogy of dust and its persistence in the atmosphere (Tegen and Fung 1995; Sokolik and Toon 1996; Okin 2002; see also chapter 9). Loss of vegetation lowers the infil-tration of rainfall, leading to higher runoff losses of rain water, greater losses of soil nutrients, and the persistence of regional desertification (Abrahams et al. 1995; see also chapters 5, 6, and 7).


• general research objectives now are (1) to understand and explain historic landscape scale dynamics characteristic of arid lands, (2) to understand current landscape structure and function, and (3) to predict future dynamics, including those of managed landscapes (Peters and Havstad 2005). All of these objectives still provide a base from which to develop strategies to manage arid lands and restore degraded areas


• These descriptions and under-standings should translate to other arid ecosystems around the world.


• 02 Regional Setting of the Jornada Basin


• Southern New Mexico consists of C3 shrubs and C4 grasses in the lower ele-vations surrounded by C3 woodlands and juniper savannas in the higher elevations (Dick-Peddie 1993


• boundary most widely used is based on a de Martonne aridity index of 10 (Schmidt 1979)


• Erosion of the steeply tilted mountain ranges has been the main source of sediment for the filling of the intermontane basins.


• the water table is 76 m (250 feet) or more deep in some places in the Jornada Basin but as shallow as 1 m near the river on the modern floodplain (King and Hawley 1975).


• located in a tectoni-cally active zone with great topographic diversity is a characteristic that the Chi-huahuan Desert shares with other North American deserts (Mojave, Sonoran, and Great Basin), the Monte Desert of South America, and deserts of the Middle East. These deserts contrast with the tectonically stable Thar Desert of India/Pakistan, the Sahara Desert, and Australian deserts (Cooke et al. 1993)


• shal-low marine and shoreline sandstone in the basal formation (Bliss sandstone) and an upper Devonian formation enriched in dark shales (Percha shale) deposited on a poorly oxygenated sea floor. Paleozoic rocks is char-acterized by numerous unconformities that represent erosion during periods of sea-level fall.


• Meter-scale interbedding of marine and nonmarine rocks was probably a response to global sea-level changes driven by growth and shrinkage of continental ice sheets in Gondwana-land (Mack and James 1986). The presence of calcic paleosols, widespread gyp-sum precipitation, and a paleoflora dominated by gymnosperms suggest that Per-mian paleoclimate in southern New Mexico was relatively dry (Mack et al. 1995).


• Mesozoic Rocks, Upper Cretaceous, rocks are composed of interbedded marine and nonmarine sandstones and shales deposited within and adjacent to theWestern Interior Seaway. Deposition of dark, organic-rich shales suggests the floor of the seaway was poorly oxygenated. A subhumid to humid paleoclimate is suggested, indicative of an open canopy forest


• McRae formation consist of silicified tree stumps


• paleobotanical evidence suggests that early in the history of deposition of the McRae formation, the paleoclimate was warm and subhumid with little seasonal variation in precipitation and supported a subtropical to para-tropical open-canopy forest.


• By the end of McRae deposition, however, the pa-leoclimate became drier and perhaps more seasonal, as indicated by the presence of calcic paleosols with vertic features (Buck and Mack 1995).


• Calcic paleosols and gypsum in the Love Ranch formation suggest a relatively dry climate in early tertiary (Seager et al. 1997).


• Traditionally, the history of the Rio Grande rift has been divided into two phases, an early phase and a late phase that began in the latest Miocene or Pliocene and continues to the present (Seager 1975; Seager et al. 1984).


• [ancestral rockies]


• [history separated into 2 phases, early volcanic activity w/o a river and then] Miocene or Pliocene time and continues to the present day and is re-sponsible for producing the modern topography.


• Up-lifts up to 5 m have taken place within the last 1,000 years along the Artillery Range Fault on the east side of the Organ Mountains (Gile 1986, 1994).


• Ancestral rio grande reached southern New Mexico about 5 million years ago (Mack et al. 1993, 1996, 1998a, 1998b; Leeder et al. 1996)


• Ancestral Rio Grande was in the Jornada Basin about 1.6 million years ago (Mack et al. 1996).


• soils of the JER La Mesa are estimated to range in age from about 0.78 to 2.0 million years


• Jornada Basin area has been occupied by humans since the latest Pleistocene, perhaps before the Clovis dates of 12,000 years ago (MacNeish and Libby 2004).


• (1) Paleo-Indian (12,000 to 7,500 years B.P.). This group, which is subdivided into Clovis and Folsom people, were hunters who subsisted on bison, mammoth, and limited foraging.


• (2) Chihuahua Archaic (8,000 to 1,750 years B.P.). These people were hunters and gatherers who migrated through veg-etation zones using available resources.


• (3) Jornada Mogollon (AD 400 to 1400 or 1450). These people were pit house-dwelling agriculturalists who made brown-ware ceramics. In the later stages, the people lived in aboveground adobe structures of contiguous rooms and made brown-ware ceramics decorated with red and black paint.


• (4) Masons (1450 to present). These people were pos-sibly the descendants of the Jornada Mogollon. They intermarried with refugees from northern pueblos after the Pueblo Revolt in 1680.


• Climate during the late Pleistocene glacial maximum in New Mexico, approx-imately 20,000 years ago, is interpreted as being cooler than today by 5-7C (Phillips et al. 1986).


• Packrat midden records from limestone cliffs of the Hueco Mountains near El Paso indicate a unidirectional change in that environment from (1) oak-juniper woodland in the early Holocene to (2) desert grassland in the middle Holocene to (3) desert scrub in the late Holocene (Van Devender 1990).


• In the lower elevations of the piedmont slopes, soil-geomorphic and fossil pollen records indicate (1) grassland in the early Holocene, (2) desert scrub in the middle Ho-locene, and (3) a return of grassland with intermittent periods of desert scrub in the late Holocene (Freeman 1972; Gile et al. 1981; Buck and Monger 1999; Monger 2003).


• Landforms of the Jornada Basin 02


• (1) the mountains and hills, (2) piedmont slopes (bajadas), (3) basin floors, and (4) Rio Grande Valley


• Jornada basin floor typically has a gradient less than 1%


• Hills rise less than 1,000 ft (305 m) above piedmont slopes, whereas mountains are higher. Both typically have slopes steeper than 15% Peterson 1981)


• White Sands National Monument on the eastern side of the San Andres Mountains.


• Conclusions 02


• During most of this time, the climate was dry enough that car-bonate occurred in paleosols (Mack et al. 1994c; chapter 4). Yet the climatic swings were high enough that during glacial maxima lakes formed in southern New Mexico in general (Hawley 1993) and the Jornada Basin in particular (Gile 2002).


• C3 woodlands in the mountains of the Jornada Basin site, C3 shrublands in the lower and drier areas and C4 grasslands in between


• erosion paralleled the decline of grasslands as progressive amounts of bare ground associated with increasing shrublands left soil unprotected and vulnerable to detachment and transport by running water and strong wind.


• 03 Climate and Climatological Variations in the Jornada Basin


• Longer-term patterns are controlled spatially by factors such as large-scale circulation patterns and basin and regional orography and temporally by the large-scale fluctuations in atmospheric and oceanic circulation patterns.


• overall climate of the basin can be defined according to the Ko¨ppen clas-sification as being cool and arid, belonging to the midlatitude desert zone (Bwk).


• Annual Patterns 03


• average rainfall for the Jornada headquarters between 1915 and 1995 is 245.1 mm


• minimum recorded value for a complete year is 77.0 mm, which occurred in 1953, with the maximum of 507.2 mm falling in 1984.


• average annual temperature between 1915 and 1993 is 14.7C


• minimum average annual temperature was 13.54C in 1987, and the maximum average annual temperature of 16.25C oc-curred in 1954.


• [evaporation] between 1953 and 1979, with values ranging from 1,565.2 mm (1976) to 2,832.6 mm (1971) and with a mean value of 2,204.1 mm


• always a large moisture deficit (evaporation minus precipitation) averaging 1,960.3 mm.


• Table 3-1. Long-term (1915-95) averages, minimums, and maximums [pg 47 pdf 64]


• Monthly and Seasonal Patterns 03


• Jornada headquarters data show a peak in rainfall between July and October, the maximum being in August, with a much smaller secondary peak in November to February (table 3-1)


• [summer rains from Gulf of Mexico as thunderstorms with lots of rain, winter months small rains from the Pacific]


• Average temperatures are at their lowest in January with a mean value of 3.78C. The lowest recorded value of -0.61C occurred in January 1919, the following January the average temperature reached its maximum of 8.33C.


• Peak average monthly temperatures occur in July when the average is 26.03C, with a minimum recorded value of 23.89C (1962) and a maximum of 28.60C (1951)


• Average January minimum temperatures are -5.99C, the lowest recorded value being -13.17C in 1963


• Maximum daytime temperatures average 13.5C in Jan-uary with a range from 6.61C (1919) to 17.94C (1920) and rise to an average of 34.96C in July. The coldest July on record was in 1916, when the maximum daytime temperature only reached 31.59C, and the warmest July saw mean day-time temperatures of 38.15C in 1980. Minimum temperatures peak at 17.11C in July, although the observed range of values is from 12.39C (1963) to 20.27C (1935).


• The onset of frosts starts on average on October 22, although dates as early as September 14 (1959) and as late as December 1 (1932)


• [last frost] average date is April 29, the earliest date is March 20 (1990), and the latest is June 10 (1963)


• [avg 128 frost free days]


• Evaporation rises rapidly in the first half of the year, peaking at average figure of 323.82 mm in June, before falling more slowly in the second half of the year. Evaporation thus peaks before either temperature or rainfall.


• A positive water balance only occurred for seven months in the recording period from January 1953 to August 1979.


• Average monthly wind speeds are highest in April with a value of 12.4 km/h, declining to a value of 7.9 km/h in August (figure 3-3)


• second maximum of 8.8 km/h in November. The least windy month is December, when the average velocity is 7.6 km/h. In terms of the average peak gust, there is a less well-defined annual pattern, although peaks occur in April (78.6 km/h) and July (78.8 km/h). Thus the summer months have lower average wind speeds, but they have important gusts, usually relating to local atmospheric convection in the af-ternoons, leading to the characteristic dust devils


• dominant direction of the peaks gusts is from the WSW, which occurs in seven months from the end of October through May. January and April have similar directions with peaks arriving from the west and southwest


• no dominant wind direction throughout the month of July [wind shifts, onset of monsoon]


• Solar radiation is asymmetrically distributed through the year, rising rapidly to a peak value in May (671.1 MJ/m2 ) and then declining more slowly to a minimum in December (244.5 MJ/m2). The average annual solar radiation re-ceived at the LTER weather station is 6,250.1 MJ/m2


• at the JER headquarters, on average, 61% of the annual rainfall occurs in these summer months


• The intensity of rainfall is significant in producing runoff and therefore in its role in distributing water through the basin catchment. Relative to soil infiltration rates, the rainfall intensity determines the extent of runoff during rainfall events of the same magnitude.


• The majority of events (55%) recorded at the LTER weather station last for an hour or less


• median hourly rainfall intensity is 0.76 mm/h, with an average of 1.7 mm/h


• [rainfall] Oscillations on a 3-year cycle are the most com-mon, followed by the 64-year cycle


• Effects of the El Nino Southern Oscillation 03


• Dahm and Moore (1994) found significant differences in winter (October to May) rain-fall, with El Nino years having about 1.5 times the medial rainfall and La Nina years having about 0.5 times the medial rainfall.


• no signif-icant difference observed in summer (June to September) rainfall in either case.


• patterns of rainfall differences have been assessed for the Jornada using the same definition of El Nino years (1919, 1926, 1940, 1941, 1942, 1952, 1958, 1964, 1966, 1973, 1978, 1983, 1987, 1992, and 1993) and La Nina years (1918, 1939, 1950, 1951, 1956, 1971, 1974, 1976, and 1989) as Dahm and Moore (1994).


• On average, El Nino years have 1.13 times and La Nina years 0.84 times the annual rainfall in medial years. The respective ratios for winter rainfall are 1.59 and 0.63 and for summer rainfall, 1.02 and 0.84. It is possible that the La Nina events in 1950, 1951, and 1956 were at least partly responsible for the period of dry weather in the 1950s, although the earlier onset (as discussed) suggests that it was probably not the principal mechanism.


• There seems to be a spatial pattern in the extent to which El Nino and La Nina events affect the rainfall within the basin. The effects in El Nino years are particularly pronounced in the higher altitude


• fttp://daac.gsfc.nasa.gov


• Although Diaz and Kiladis (1992) showed that there was a significant decrease in December-Feb-ruary temperature in parts of New Mexico and Texas relating to ENSO events, the Jornada Basin is on the edge of the zone they delimited as being significant.


• North Atlantic Oscillation (NAO) Pacific North America Index (PNA)


• fluctuations exist on cycles extending from 3 to 64 years. Precipitation fluctuations are reinforced by the occurrence of ENSO events, with significant increases in winter precipitation in El Nino and significant decreases in La Nina years. To a certain extent, these fluctuations are reinforced by tele-connections with the NAO and PNA signals. The coincidence of these larger scale teleconnections with other cycles in the climate may serve to amplify the varia-tions, for example, the repeated La Nina events in the 1950s superimposed on the preexisting drought signal.


• It is possible, then, that longer term fluctuations in vegetation may be triggered by combinations of the various climatic conditions, controlled in the winter season by the ENSO, PNA, and NAO signals and in the summer by the movements of the Intertropical Convergence Zone and the development of monsoonal conditions and their inter-play with auto variations in the local climate


• Although the mechanisms behind most of these remain unexplained, there is evidence for important quasi-periodic fluctuations relating to variability in global circulation patterns, particularly those represented by the Southern Oscillation and to a lesser extent the NAO and the PNA signal.


• The Southern Oscillation is particularly important in controlling the amount of winter precipitation, with significantly wetter conditions occurring in El Nino years and significantly drier conditions occurring in La Nina years. Ac-cording to Neilson (1986), the El Nino years should thus favor shrub vegetation and the La Nina years grass vegetation.


• Wainwright (2005) reviews evidence that suggests that this climatic regime seems to have been in place for around the last 4,500 years


• The climate seems to have oscillated between extremes of wet and dry conditions over this time period, although some of the records suggest that the present century has seen the largest magnitude extremes.


• There is evidence for oscil-lations of even wetter conditions superimposed on this increase both in the Jor-nada Basin and elsewhere in the Southwest, although this seems to have been more pronounced in the Sonoran Desert than the Chihuahuan Desert. The mech-anisms of these changes are unknown.


• Estimates of temperature change vary from about 2#Cto6#C cooler than present at the late glacial maximum. It is likely that the patterns of seasonality were highly different from those at pres-ent


• www.ambiotek.com/advances/advemma/indivs/wainwright.pdf


• 04 Soil Development in the Jornada Basin


• caliche depends on the age of the soil, 1918 investigation by J.O. Veatch


• The 1918 Map 04 [fig 4.1, pg 83, pdf 100]


• West Well Gravelly Sand (WGS) = caliche at 2-6 ft deep, 6 ft thick, threeawn (Aristida)grama (Bouteloua) association with no brush except for a few mesquite (Prosopis glandulosa) in wind-eroded areas


• Jornada Red Loamy Sand (RLS) = caliche was not expose, lack-tailed prairie dogs (Cynrmys ludovicianus), black grama (Bouteloua eriopoda) and absence of brush, best forage land,


• Jornada Sand (RLS[W]) = Eroded RLS, shifted by wind into low mesquite hummocks and ridges., caliche at 6-10 ft deep, exposed in some places,


• Lake Bed Clay (LC) = highly calcareous, chocolate red or grayish clay with scattered gypsum crystals overlying pure gypsum beds at 10 ft, burrograss (Scleropogon brevi-folius) and tobosa grass (Pleuraphis mutica), entire absence of soapweed and shrubs present on neighboring sandy soils


• Gyp-sum Soil (GY.S) = small areas where gypsum was at or very near the surface, covered only by a thin veneer of silt or very fine sand with only scant amounts of dropseed (Sporobolus) and Mormon tea (Ephedra)


• Middle Well Sand (SG) = was deep, loose sands that covered substrata of gypsum. This unit was then nearly barren except for some remaining threeawns and black grama.


• Jornada Gray Sand (GSL) = overlaid gypsum, transitional from burrograss and tobosa grass of the lake bottoms to considerable black grama, threeawn, and soapweed (Yucca elata) on the slopes.


• piedmont slopes contained four units:


• clay or adobe soil (A) = boundary of the piedmont slope and basin floor, "lime cementation.", predominantly burrograss, tobosa grass, and tarbush (Flourensia cernua)on the flats with soaptree yucca, honey mesquite, Mormon tea, and creosotebush (Larrea tridentata) on the ridges


• Goldenburg Sands (GS) = wind-laid deposits, younger caliche, mesquite, with common amounts of sand sage (Artemisia) and saltbush (Atriplex), and only sparse growth of dropseed and threeawn grass


• Jornada Clay Loam (RSL and SCL) = was used to map various intermediate pied-mont slope positions. The unit had a range of textures, degrees of lime carbonate, and vegetation, which consisted of brush (tarbush, creosotebush, and mesquite) and grasses (burro, tobosa, grama, and threeawn)


• Middle Tank Gravelly Soils (GL) = higher on piedmont slopes, soils contained coarse rock detritus that in places were cemented into conglomerates that restricted root penetration. Vegetation was de-scribed as a creosotebush-black brush (assumed to mean tarbush) association of low density, which attributed to the low moisture content of the soil.


• Foothills (GSS) = were described as consisting of thin, silty, residual soils, mesquite and soaptree yucca vegetation


• Mountain Slope Soils (GL and SS) = consisted of very thin, stony, yet dark soils, mixed grasses, mountain mahogany (Cercocarpus) , and other shrubs occurred on the slopes while pin ˜on (Pinyon), juniper (Juniperus), and oak (Quercus)


• The 1963 Map 04 [pg 85, pdf 102]


• 1980 Dona Ana Map 04


• Table 4-1. Correlation of 1918 and 1963 soil maps of the Jornada Basin area and abbreviated descriptions of 1963 mapping units [pg 87 pdf 104] (1) carbonate horizons are parallel to the land surface; (2) carbonate horizons have upper boundaries within several inches to about 2 feet of the soil surface; (3) carbonate horizons have distinctive morphologies that show lateral continuity and differ markedly from morphologies of overlying and underlying horizons; (4) carbonate horizons occur between horizons containing little or no carbonate; (5) carbonate horizons occur across sediments of various compositions and textures; and (6) carbonate horizons form in a developmental sequence related to time (Gile et al. 1965). <P>Ten years of dust measurements revealed that calcareous dust fell ubiquitously on the landscape, ranging from 0.2 g/m2/yr in a grassy basin floor area to 1.1 g/m2/yr in a sandy bare-ground area (Gile and Grossman 1979). Meanwhile, rain as a source of Ca became recognized as a more important source of Ca than dust. Chemical analysis of rain by Junge and Werby (1958) and Lodge et al. (1968) revealed that Ca from rainwater could produce an estimated 1.5 g CaCO3/m2/yr, assuming 200 mm an-nual rainfall. Therefore, if ample bicarbonate is generated by roots and microbes, carbonate resulting from Ca in rain could be roughly two to three times greater than carbonate resulting from calcareous dustfall (Gile et al. 1981).


• Factors that obliterate argillic [phyllosilicate clay] horizons include (1) landscape dissection and erosional truncation of argillic horizons, (2) engulfment of argillic horizons by pedogenic carbonate, and (3) faunal mixing in which tun-nels and mounds made by kangaroo rats(Dipodomys), badgers (Taxidea), and termites (primarily Gnathamitermes) destroy the fabric of argillic horizons (Gile 1975a).


• SCS (now the Natural Resources Conservations Service, NRCS) NRCS Web site (http://soils.usda.gov


• [late 1800, early 1900] Russian concept that soils are the evolutionary product of five fac-tors - topography, climate, parent material, biota, and time.


• [additionally] Simonson (1959): ad-ditions, transfers, transformations, and removals (figure 4-5) [act on soil development]


• Desert soils in the Jornada Basin have low concentrations of soil organic matter [SOM]


• Even in A horizons most concentrations are only 0.1-0.3% organic carbon. On the high end, soils above elevations of about 1,524 m (5,000 ft) can have up to 1.3% organic carbon in A horizons (Gile et al. 1981). These soils have enough organic matter to classify as Mollisols, rather than Aridisols or Entisols. Downslope, these soils change from Mollisols to Aridisols.


• A horizons retain a dark color (giving a false impression of high organic content)


• Other soils containing relatively high levels of organic matter are located in topographically low areas that receive water from hill slope runoff. In these soils, organic carbon can be as high as 2.21% (Herbel et al. 1994).


• This is the result of greater plant densities supported by increased water supply and an increased clay content that curtails organic matter decomposition (Deng and Dixon 2002).


• soils developed in limestone alluvium have a tendency to contain higher amounts of organic matter than neighboring soils formed in igneous al-luvium (Grossman et al. 1995).


• On the low end, some sandy soils barren of vegetation in the Jornada Basin contain as little as 0.1% organic carbon


• As in the Mojave Desert (Schlesinger 1985), roots (Gallegos 1999) and microorganisms (Monger et al. 1991a) play a role in precipitating pedogenic carbonate


• soils of the southeastern region of the JER, which are of early Pleistocene age (Mack et al. 1996), have pedogenic carbonate amounts that reach 223 kg C/m2, a value that equals some of the highest concentrations of organic carbon in peat-bog Histosols (NRCS Web site 2005)


• to describe soil carbonate in arid and semiarid soils: caliche, calcrete, croute calcaire, tosca, caprock, crust, calcic horizons, and petrocalcic horizons (Gile 1961; Goudie 1973; Dregne 1976; Soil Survey Staff 1999).


• carbonate accumulation have been described as filamentary, con-cretionary, cylindroidal, nodular, plugged horizons, and laminar horizons (Gile 1961; Gile et al. 1966). These carbonate accumulations range from nonindurated, which slake when placed in water, to very strongly indurated, which do not slake in water and cannot be scored with a knife


• On the low end, soil horizons with carbonate filaments can contain as little as 1% CaCO3 and, depending on texture, have a bulk density of about 1.68 g/cm3 and an infiltration rate of 12.4 cm/h (Gile 1961). On the high end, laminar horizons contain as much as 93% CaCO3,have a bulk density of 2.22 g/cm3 , and have an infiltration rate of 0.1 cm/h.


• Carbonate is an important indicator of soil age because progressively older geomorphic surfaces contain progressively greater amounts of carbonate. Over time,


• Calcium carbonate is known to have important chemical influences on plant growth by its control on pH, phosphorous, and mi-cronutrient availability


• Carbonate crystals are generally in the size range of coarse clay to fine silt, approximately 1-10 microns (Monger et al. 1991b), which increases surface area and microporosity of the horizon im-pregnated with carbonate. For example, soil-water release curves for carbonate nodules revealed that the nodules store about twice the water as adjacent soil not impregnated with carbonate (Monger 1990). Hennessy et al. (1983b) measured water properties of caliche in the Jornada Basin and found that caliche absorbed appreciable quantities of water and retained it for extended periods, which may have contributed to certain areas of black grama grass underlain by caliche sur-viving the 1950s drought (Herbel et al. 1972).


• creosotebush on sites underlain by caliche showed less water stress when rainfall was low than sites without caliche (Cunningham and Burk 1973).


• As clay content increases, water storage, nutrient storage, shrink-swell, and hardness generally increase, whereas erodibility and air permeability generally decrease.


• [most clay, 69% in playas, 20-30% water. Least clay, 5% in coppice dunes, 5% water]


• Alluvial activity in the Jornada Basin consists of two processes: erosion and sed-imentation. Water erosion is prominent on the piedmont slopes [but is everywhere]


• In general, erosion in the Jornada Basin produces sediments that are not removed by water from the basin. The exception is the Rio Grande Valley, where the drainage system is integrated with the river.


• Lakes of the Jornada Basin have probably been dry since the end of early Ho-locene (8,000 years ago) when similar lakes in the northern Chihuahuan Desert became extinct (Hawley 1993).


• eolian activity consists of two categories: erosion and sed-imentation. [wind erosion can carry particles out of system]


• If mesquite shrubs are present, a portion of this material is trapped and coppice dunes form (Gile 1966b). [wind erosion]


• Kangaroo rats can penetrate into carbonate horizons and bring carbonate fragments to the surface (Anderson and Kay 1999), where they are apparent in aerial photographs. In some cases, termites are agents of carbonate movement.


• A typical value of 500 mm (20 inches) [of rain] has been used as the general boundary between soils with carbonate and soils without carbonate (Birkeland 1999).


• the climate in the Jornada Basin has probably been arid, semiarid, or at most subhumid for at least the past 500,000 years


• depth of pe-dogenic carbonate is proportional to annual rainfall, that is, greater precipitation corresponds to greater depths to the top of the carbonate horizons (Jenny and Leonard 1934; Arkely 1963; Gile 1975c)


• However, erosion, runoff, and run-in can confound this general relationship, and it has recently been shown that a statistically significant correlation does not exist between carbonate depth and rainfall, especially for shallow carbonate in arid and semiarid climates (Royer 1999).


• shifts in 13C/12C ratios indicated a change from C4(grass) to C3(shrub)


• stage I carbonate fil-aments can develop within 100 years, whereas stage II nodules take about 8,000 years to form (Gile et al. 1981). Stage III and IV petrocalcic horizons might take 75,000 years to form depending on texture.


• Organic carbon, which has important influences on nutrient cycling and aggregate stability, ranges from 0.1% weight in sand dunes to 2.2% weight in soils in depressions that receive run-in water.


• Erosion and sedimentation working in concert with Cenozoic tectonic extension has produced the modern terrain of the Jornada Basin.


• 05 Patterns and Controls of Soil Water in the Jornada Basin


• In arid and semiarid regions, water is typically thought to be the most limiting resource to biological activity (Noy-Meir 1973), though colimitation by water and nitrogen may be a more general rule (Hooper and Johnson 1999


• availability of water affects plant productivity, microbial activity, activity of biological soil crusts, nutrient cycling, and organic matter decomposition. It also directly and indirectly affects soil erosion, chemical weathering, and carbon-ate formation.


• interactions between rainfall patterns, soil characteristics, temperature, and topography are critical to predicting ecosystem responses.


• At local scales when time is substituted for space, precipitation appears to be a poor measure of water availability and productivity because of the complex effects of differences in rainfall frequency, timing and magnitude, landscape position, soil texture, soil structure, macropores, microrelief, and feedbacks between the veg-etation and hydrologic processes such as stem flow, infiltration, percolation, and runoff. Soil moisture is a more direct indicator of available water for biological activity, but accurate data are rarely available at relevant spatial scales due to measurement and scaling limitations (Williams and Bonnell 1988).


• Southwestern desert regions of North America are characterized by a bimodal pattern of rainfall where precipitation is received in both the winter months and during the summer growing season. The ratio of summer to winter rainfall varies across the desert regions.


• Jornada Basin is an area where shrubs have invaded and dramatically changed the landscape and ecosystem processes of areas formerly dominated by grasslands.


• Causes of Soil Water Heterogeneity 05


• Climate 05</b>


• Winter rainfall often percolates to greater depths because lower rain-fall intensities allow more of it to enter the soil and because lower evapotran-spirational demands increase the probability that upper soil layers will already be near field capacity when precipitation events begin. Summer rainfall is character-ized by localized convective storms, which are generally of short duration and high intensity. These rainfall events generally do not percolate to deep soil layers because of high evaporation and transpiration rates and an increased likelihood of surface runoff.


• This pattern can sometimes be reversed in the lowest landscape positions, which benefit from run-in during more intense summer storms


• Landscape Position and Soil Properties 05


• Rain gauge records show that rainfall increases with ele-vation and that there is also a high level of variability at the same elevation


• Landscape position is also a good predictor of soil texture, which affects in-filtration capacity, water holding capacity, and bare-soil evaporation rates.


• gen-eral, infiltration capacity is highest on the sandy basin soils, intermediate on the loamy alluvial fans and fan piedmont areas, and lowest on the fine-textured soils of the alluvial flats and lake plains.


• calcic horizons can have a significant effect on deep percolation of water due to their generally low hydraulic conductivity.


• Volumetric water content is the proportion of the soil volume that is occupied by water. The maximum volumetric soil water content after gravity drainage is referred to as field capacity.


• Plant available water-holding capacity is generally highest in intermediate-textured soils (loamy) and lowest in sandy soils. It is reduced by the presence of rocks but may actually be increased by the presence of calcium nodules in sandy soils (Hennessy et al. 1983a).


• In arid and semiarid ecosystems, soil surface structure is generated by litter decomposition under plants, microbiotic crusts in plant interspaces, and macroinvertebrates in both plant and interspace microsites (Herrick and Wander 1998). Repeated cycles of wetting and drying help form aggregates, and freeze-thaw cycles can also be important. Macroinvertebrates, especially ants and termites, are extremely important in the Chihuahuan Desert for increasing infiltration through the formation of macropores (Elkins et al. 1986; Herrick 1999).


• Development of soil structure below the soil surface is similar to more humid environments, where root decomposition and associated soil biotic activity dominate; however, ants and termites replace earthworms as the dominant macropore-forming organisms in these environments (Herrick 1999). Soil structure-forming processes are self-reinforcing as improved soil structure facilitates greater water infiltration and retention, leading to higher litter production in subsequent years


• Vegetation 05


• Increased plant basal cover can increase water percola-tion depth by slowing runoff, and canopy cover reduces raindrop erosivity and therefore limits soil surface degradation


• Shading by plant canopies and increased litter cover below plants reduces soil surface temperatures and re-sultant evaporation


• tends to be a greater occurrence of ma-cropores under plant canopies


• hydraulic lift, where deeply rooted plants redistribute water from wet, deep soil layers to drier, shallow soil layers (Richards and Caldwell 1987; Caldwell et al. 1998)


• it has been found that after rainfall events, roots of some species transfer water from wet shallow soil layers to drier deeper soil layers (Burgess et al. 1998, 2000; Schulze 1998; Smith et al. 1999; Ryel et al. 2002).


• shifts in community com-position may contribute to variation in soil moisture.


• black grama (Bouteloua eriopoda) sites may have lower runoff rates due to better soil structure associated with more continuous plant cover (Neave and Abra-hams 2002; Schlesinger et al. 2000), generally lower slope, and coarser-textured soils.


• Less variability in soil water at and below 90 cm indicates that rainfall rarely infiltrates to these depths


• The neutron probe data show con-sistent soil water at depths of greater than 1-2 m in this arid system, where groundwater is approximately 100 m below the soil surface.


• Rainfall events preceded peaks in soil moisture by 4 weeks at 30 cm depth and 11 weeks at 130 cm depth.


• The movement of water to 130 cm depth in nonplaya soils with an intermediate amount of clay and an increase in clay content with depth took four to six weeks.


• Playa soil reached maximum infiltration at all depths in 8 weeks, but all other sites took between 14 and 24 weeks to reach maximum water content.


• Deep infiltration can occur through relatively carbonate-free "pipes," which form where the petrocalcic horizon is penetrated by animal burrows, strong roots, or anything that allows water to preferentially move through the horizon.


• The Role of Seasonal Precipitation 05


• Soil water data from NPP sites indicate that recharge of deep soil moisture takes place in above-average precipitation years, especially if this precipitation falls during winter months.


• storms of low intensity and longer duration that enhance infiltration. In addition, evaporative losses are less due to lower temper-atures, and transpiration losses are reduced due to the majority of plants being dormant in these cooler months.


• Playa sites are in topographic de-pressions that have substantial run-in in both seasons. Substantial run-in has been observed to cause prolonged flooding


• Although rainfall is greatest during the summer monsoon months, especially July and August, shallow soil depths are generally wetter in winter months than in summer (figure 5-6)


• probability of wet soil was as great or greater during winter months than during the summer growing season on both loamy sands of the basin floor and clay loams of the fan piedmont (Herbel and Gibbens 1987, 1989)


• It appears that there is shallow water in late winter and early spring that is potentially not fully used by plants. This may represent an unused resource space that could be at risk for invasion by winter annuals, such as red brome (Bromus rubens) that has invaded much of the Mojave, Great Basin, and Sonoran Deserts (Hunter 1991) or early growing, warm-season perennials, such as Lehmann lovegrass (Eragrostis lehmaniana).


• Fairly large rain events are needed during the summer months to change soil moisture at depths greater than 10 cm, the importance of smaller rainfall events on near-surface soil layers, sandy soils (Reynolds et al. 1999b), and vegetation is poten-tially not trivial.


• Small storm events account for a large proportion of precipitation events in semiarid regions (Sala et al. 1992; Hochstrasser et al. 2002).


• Blue grama (Bouteloua gracilis)was found to have a significant increase in leaf water poten-tial and conductance in response to simulated 5 mm rainfall events on the short-grass steppe in Colorado (Sala and Lauenroth 1982). The importance of moisture in near-surface soil layers is likely to be extremely important to shallow-rooted species.


• Vegetation and Soil Moisture 05


• Consistent patterns of soil chemistry and vegetation type have been found to exist for four vegetation groups on the Jornada Basin (Stein and Ludwig 1979).


• Seven distinct zones of soil water were found that were in general well correlated with vegetation and soil texture. The exception to the general pattern of distinct zones was in the vegetation zone, which spanned a majority of sampling stations (stations 15-60) and was histori-cally dominated by black grama, but at the time of the study, black grama cover was greatly reduced. The area was dominated by threeawn (Aristida longiseta), a perennial grass, soaptree yucca (Yucca elata), and globe mallow (Sphaeralcea subhastata).


• Gypsum block data showed substantial changes in soil moisture under different vegetation on the same soil type.


• There is a general relationship between total profile soil water content and soil surface texture; finer surface soil textures were associated with higher total water content.


• analyses strongly suggest that community composition and surface soil texture are not the only controlling factors for soil water and that landscape linkages must also be considered.


• resources which become increasingly concentrated below and around shrubs while the bare interspaces between shrubs have reduced amounts of litter, nutrients, and inputs from animal activity (Schles-inger et al. 1990; Cross and Schlesinger 1999).


• Dune soils with established mesquite had greater infiltration, greater hydraulic conductivity, and less evaporation than interdunal soils. How-ever, gravimetric water content at 15 cm did not differ between dunes and inter-dunes and was closely related to rainfall. At 30 cm in depth water content, mea-sured with a neutron probe every two weeks, was significantly less in vegetated dunes than in interdune areas. Similarly, preliminary analysis of NPP soil water data indicates that water content measured in neutron probe tubes in interspaces of the mesquite sites is consistently greater than water contents measured in neu-tron probe tubes under mesquite canopies (Snyder and Mitchell unpublished data).


• Because neutron probes measure water stored in nearby plant roots, as well as nearby soil, actual soil water content may potentially be even lower under shrubs (Hennessy et al. 1985). Although water may infiltrate faster and deeper below shrubs, limited evidence suggests there is not more water stored in shrub islands. This is likely due in part to plants having greater root volume in canopy areas and, consequently, using more water from the canopy area. Shrub islands may be areas of greater infiltration and percolation, but plants may quickly take up this water


• Conclusions 05


• Patterns of soil water appear to vary greatly within a community type as well as across the landscape, and the relationship to community type and production appears to be complex.


• shallow soil water is highly variable and cannot be completely explained by temporal differences in precipitation and spatial differences in soil texture. There is water at depth in these systems, and this water is less temporally variable. Recharge of unsaturated soil moisture at depth is a poorly understood process but appears to occur mostly in wetter-than-average winter months. There is little evidence to support predictions that shrub islands have more stored soil water than barren interspaces, but these islands do appear to be areas of greater infiltration.


• Sandy loams appear to be a particularly susceptible soil type


• 06 Nutrient Cycling within an Arid Ecosystem


• Low quantities of soil nitrogen limit plant growth in the Chihuahuan Desert (Ettershank et al. 1978; Fisher et al. 1988; Lajtha and Whitford 1989; Mun and Whitford 1989) and in other deserts of the world (Wallace et al. 1980; Breman and de Wit 1983; Sharifi et al. 1988; Link et al. 1995). Indeed, although deserts are often regarded as water-limited systems, colimitation by wa-ter and N may be the more general rule (Hooper and Johnson 1999; Austin and Sala 2002).


• after water, N is the most likely resource to de-termine the plant productivity of this ecosystem. [JER]


• islands of fertility


• A nearly random distribution of extractable N was found in grassland soils, but in areas dominated by creosotebush (Larrea tridentata) the distribution of soil N was patchy at a scale close to the average size of shrubs (figure 6-1).


• The patchy habitat created by shrubs also determines the biodiversity of ani-mals at higher trophic levels, including lizards and birds (Pianka 1967; Naranjo and Raitt 1993). Patchy distributions of soil microbial biomass (Mazzarino et al. 1991; Gallardo and Schlesinger 1992; Kieft 1994; Smith et al. 1994; Herman et al. 1995), nematodes (Freckman and Mankau 1986), and microarthropods (Santos et al. 1978) reflect the heterogeneous distribution of soil nutrients in desert shrub-lands. Indeed, most ecosystem function in shrub deserts is localized under vege-tation, whereas the adjacent shrub interspaces are comparatively devoid of biotic activity (see chapter 12).


• Greater microbial activity under shrubs is manifest in high rates of N miner-alization and nitrification (Charley and West 1977; Mazzarino et al. 1991; Smith et al. 1994). These microbial processes have the potential to produce gaseous by-products - NH3, NO, N2O and N2 - that are lost to the atmosphere.


• in most cases, the shrubs act to conserve N by its immobilization in the litter and microbial biomass of soil mounds (Peterjohn and Schlesinger 1991; Schlesinger and Peterjohn 1991; Gallardo and Schlesinger 1992; Zaady et al. 1996)


• Total ground cover is the most important variable influencing runoff and sediment production on desert range-lands in southern New Mexico (Wood et al. 1987) and other arid and semiarid regions (Zobisch 1993). In the Jornada Basin, vegetation aerial cover can be up to 50–60% in grasslands versus 30% in creosotebush and mesquite (Prosopis glandulosa) shrublands (Schmidt unpublished data). When shrubs replace grass-lands, the rate of erosion increases and the surface soil materials are progressively lost from the barren shrub interspaces, especially for sand-textured soils (Bull 1979; Abrahams et al. 1994, 1995; Gutierrez and Hernandez 1996; see also chap-ter 9). When shrubs are widely spaced, the barren intershrub soils are also subject to wind erosion that redistributes soil materials across the landscape (Snow and McClelland 1990; Stockton and Gillette 1990; Okin and Gillette 2001).


• Fisher et al. (1988) found that the growth of creosotebush nearly doubled with experimental additions of 100 kg N/ha, with the greatest plant growth seen when N and water were added together (figure 6-2).


• During years of high winter rainfall in the Jornada Basin, the decomposition of an abundant growth of spring annuals can immobilize soil N, leading to deficiencies that subsequently limit the growth of creosotebush during the summer (Parker et al. 1984a).


• N inputs from the atmosphere average around 2.5 kg/ha/yr in the Jornada Basin (Schlesinger et al. 2000)


• The N in atmospheric deposition is supplemented by rather meager inputs, mostly less than 1 kg N/ha/yr, from asymbiotic N-fixing bacteria in soil crusts (Loftis and Kurtz 1980; Hartley and Schlesinger 2002). In the Jornada Basin, the only appreciable rates of soil fixation, ranging up to 10 kg N/ha/yr, are found in tarbush (Flourensia cernua) communities, especially in soils with a low N/P ratio (Hartley and Schles-inger 2002). Herman et al. (1993) also report N-fixing bacteria in the rhizosphere of black grama (Bouteloua eriopoda)


• Rundel and Gibson (1996), who report similar low rates of asymbiotic fixation in creosotebush habitats in the Mojave Desert of southern Nevada.


• Symbiotic N fixation is largely confined to habitats dominated by mesquite. At the Jornada, Jenkins et al. (1988) report nodules on mesquite roots from 13 m depth, complicating attempts to estimate overall N inputs to this ecosystem. Based on studies in other ecosystems, we might expect N fixation in mesquite habitats to range from 40 (Rundel et al. 1982) to 150 kg N/ha/yr (Johnson and Mayeux 1990), depending on plant cover. From measurements ofd15 N in its foliage, Lajtha and Schlesinger (1986) estimated that mesquite in the Jornada Basin obtains 48% of its N from symbiotic fixation, accounting for about 20 kg N/ha/yr.


• In the face of these inputs of N, the Chihuahuan Desert persists in an N-deficient state, owing to soil erosion by wind and water and to the microbial production and loss of N-containing gases to the atmosphere.


• Wind erosion may remove up to 14 kg N/ha/yr from mesquite habitats, assuming a soil N concen-tration of 0.1% (Gallardo and Schlesinger 1992)


• losses of 1-5 kg N/ha/yr are associated with the suspended and bedload sediments carried in runoff waters. Only a small fraction of the runoff loss occurs in forms that are available to plants. For instance, Schles-inger et al. (2000) report runoff losses of dissolved N totaling 0.15 kg N/ha/yr in black grama grasslands and 0.33 kg N/ha/yr in creosotebush shrublands. A sur-prising fraction of the dissolved N loss, nearly 70% on bare soils, is carried in dissolved organic forms (DON). Relatively little N is lost to groundwater in the Chihuahuan Desert


• Peterjohn and Schlesinger (1991) report bursts of denitrification after wetting events, potentially leading to losses of 7.2 kg N/ha/yr from these ecosys-tems.


• but much of the apparent loss may simply result in a local redistribution of soil nutrients on the landscape.


• Intrasystem Nutrient Cycling 06


• Plant nutrient uptake by Chihuahuan Desert shrubs is closely tied to the avail-ability of soil water. In watering experiments, creosotebush showed a rapid uptake of soil N, whereas mesquite showed little response (BassiriRad et al. 1999).


• N-use effi-ciency is inversely correlated to water-use efficiency, so both C acquisition and water conservation are greater in desert plants with high leaf N contents (Lajtha and Whitford 1989). However, plants with high leaf N contents are also more attractive to insect and mammalian herbivores, who enhance the return of N to the soil through their feeding (Lightfoot and Whitford 1989, 1990; Day and De-tling 1990; Frank and Evans 1997).


• Mycorrhizae may contribute to the nutrient-uptake capacity of many species in the Jornada Basin (Herman 2000). Abundant fungal endophytes with mycor-rhizal traits have been found on fourwing saltbush (Barrow et al. 1997) and black grama (Barrow 2003) in the Jornada Basin, and creosotebush is reported to harbor mycorrhizae in the deserts of southern California (Bethlenfalvay et al. 1984). The mycorrhizae on fourwing saltbush are dark septate fungi with the ability to sol-ubilize rock phosphate (Barrow and Osuna 2002).


• In the Chihuahuan Desert only a small amount of phosphorus is found in organic forms, which may be of special importance to plant phosphorus nutrition (Cross and Schlesinger 2001). Often, organic and bicarbonate-extractable P (forms easily available for plant up-take) are concentrated beneath the canopy of shrubs, whereas Ca-bound P is greatest in the shrub interspace (Charley and West 1975; Cross and Schlesinger 2001).


• Whitford et al. (1981b) noted that the rate of decomposition of plant litter in the Chihuahuan Desert was greater than predicted by the simple correlation with actual evapotranspiration, as promulgated by Meentemeyer (1978) in his seminal study comparing decomposition in various terrestrial biomes.


• Schaefer et al. (1985) found that lignin content, C/N ratio, and the lignin/N ratio in plant litter also failed to predict decomposition in the Jornada Basin, despite the success of these variables as predictors of decomposition in a variety of other ecosystems (Melillo et al. 1982). Unexpected high decomposition rates of surface litter were thought to be related to its photo-oxidation by ultraviolet light and to the abundant activity of microfauna and termites (Gnathamitermes) in desert soils.


• Johnson and Whitford (1975) found that termites consumed about 50% of the surface litter in creosote bush and mesquite communities


• Because much of the plant litter in the Jornada Basin is processed by the activities of termites, rainfall timing and amount is not as strong a predictor of decomposition in the Chihuahuan Desert (Santos et al. 1984; Whitford et al. 1986; Kemp et al. 2003) as it is in other desert ecosystems (Strojan et al. 1987)


• [faster decomposition under shrubs, hld more moisture after rains] (Whitford et al. 1980b, 1982; Parker et al. 1984a)


• decomposition of buried litter and roots, largely mediated by microarthropods (Santos et al. 1984), is faster than the rate of disappearance of surface litter, which shows greater fluctuations in moisture content (Schaefer et al. 1985).


• Litter quality, especially N content, appears to play only a limited role in determining rates of decomposition in desert habitats of the Jornada Basin (Schae-fer et al. 1985) and elsewhere in the American Southwest (Murphy et al. 1998). Experimental additions of N had little effect on the decomposition of black grama or creosotebush litter (MacKay et al. 1987a; see also Mun and Whitford 1998).


• soil microbial biomass is related to the content of soil organic carbon and extractable N (NH4 #NO3) (Gallardo and Schlesinger 1992). Fertilization with N increases microbial biomass in grassland soils, whereas additions of C have little effect. In shrublands, fertilization by C increases microbial biomass and decreases extractable N and P, which are im-mobilized during microbial growth (Gallardo and Schlesinger 1995). In many areas of the Jornada Basin, where shrubs have invaded upland grasslands, the proportional net decrease of soil organic C exceeds that for soil N, so that soil C/N ratios decrease and C becomes limiting for microbial biomass as desertifi-cation proceeds (Gallardo and Schlesinger 1992, 1995; Kieft 1994).


• Completing the nutrient cycle, the release, or "mineralization," of N from soil organic materials is closely tied to fluctuations in soil moisture. Fisher et al. (1987) found that small, frequent experimental applications of simulated precip-itation (6 mm/week) caused greater rates of N mineralization than a larger, infre-quent event (25 mm/month), followed by periods of drought.


• N mineralization, approximated from estimates of plant uptake, ranges from 28 to 64 kg N/ha/yr in creosotebush habitats (Whitford and Parker 1989)


• [mineralization of N is greater in grasslands than under mesquite b/c mesquite gets 48% of its N from its fixation]


• biomass turnover is quite rapid in Chihuahuan Desert ecosystems, with mesquite having the longest mean residence time for N in biomass - about two years.


• In all habitats, turnover of N in roots is greater than or equal to that of aboveground components, and the percentage turnover of N in roots is greatest in grasslands, where the mean residence time for N in roots is less than 1 year.


• Given similar topography, there is greater N loss through runoff in shrub-invaded systems (Schlesinger et al. 2000). These systems are more susceptible to runoff because there is less total plant cover to intercept rainfall and to prevent erosion and scouring by wind and water. With larger plants and gaps between plants, flow paths are connected over longer distances (Howes and Abrahams 2003). There are also greater gaseous losses of N in shrublands through higher denitrification rates (Peterjohn and Schlesinger 1991; Hartley 1997). Thus our comparison of the N cycle in shrubland and grassland communities shows that areas invaded by shrubs have greater N losses even though shrubland landscapes appear to sequester equivalent or greater amounts of soil N (Jackson et al. 2002).


• Desertification of Grasslands and Remediation of Shrublands 06


• In southern New Mexico, desertification is associated with the loss of grassland, dominated by black grama, and the invasion of desert shrubs, primarily mesquite and creosotebush (Buffington and Herbel 1965)


• NPP = net primary production


• On a landscape-scale, the mass of soil nutrients in shrublands is similar to or higher than that in grasslands (Kieft et al. 1998; Cross and Schles-inger 1999; Hibbard et al. 2001; Jackson et al. 2002), but the spatial distribution of the soil nutrients contrasts strongly between these communities.


• desertification is not so much associated with a loss of biotic productivity as with the redistribution of soil resources on the landscape (Schlesinger et al. 1990, 1996) that increases the scale of patchiness (Hook et al. 1991; Tongway and Ludwig 1994). Lower NPP is the expected and traditional outcome of arid land degradation, but changes in the spatial distribution of soil resources may be a more effective index of desertification.


• Resource islands develop as a function of shrub age (Facelli and Brock 2000; Shachak and Lovett 1998). Large shrub mounds are partly erosional and partly depositional features (Abrahams and Parsons 1991; Abrahams et al. 1995).


• [grasslands are more stable]


• [raindrops disipate energy in shrub canopy, leading to more deposition]


• [kangaroo rats and rodents dig and redistribute soil under shrub canopy]


• shrubs appear to "mine" nutrients from the soils of the interspace (Garner and Steinberger 1989)


• Shrubs, such as acacia (Acaciassp.) and mes-quite, which maintain symbiotic, N-fixing bacteria in their rooting system, directly contribute to the accumulation of N beneath their canopy (Garcia-Moya and McKell 1970; Gerakis and Tsangarakis 1970; Tiedemann and Klemmedson 1973; Virginia and Jarrell 1983; Lajtha and Schlesinger 1986; Wright and Honea 1986).


• infiltration rates are typically higher under desert shrubs as a result of better soil crumb structure and a lower impact energy of raindrops (Lyford and Qashu 1969; Bach et al. 1986; Rostagno 1989; Shachak and Lovett 1998; Schlesinger et al. 1999; Wainwright et al. 1999A, 2000; see also chapter 5).


• [nutrients accumulate under the shrubs and not in the spaces between. In grasslands the nutrients are evenly spaced. There is more than just erosion causing this, some other process accumulating or depleting]


• islands of fertility are not simply a remnant left after erosion (see Kieft et al. 1998).


• Concentrations of nutrients in the islands of fertility are greatest at the soil surface and attenuate with depth (Nishita and Haug 1973; Charley and West 1975; West and Klemmedson 1978; Rostagno et al. 1991).


• The depth to the peak concentration of various solutes in desert soils follows the global pattern reported by Jobba´gy and Jackson (2001) from shallowest to deepest in the order P/K/Ca/Mg/SO4=Na=Cl. This vertical pattern mirrors the hori-zontal pattern extending from shrubs to the shrub interspaces (Schlesinger et al. 1996).


• When shrubs are removed by cutting, herbicides, or fire, N, P, and other soil nutrients are lost from former islands of fertility. Elimination of the local biogeo-chemical cycle associated with shrubs allows physical processes to disperse soil nutrients across the landscape. The redistribution of N seems to be more rapid than that of P, likely due to more rapid gaseous and soluble losses of N and retention of P by adsorption to soil minerals. Thirteen years after the removal of mesquite there was a significant loss of soil N from former shrub islands, but there were no significant changes in P or S over the same period (Tiedemann and Klemmedson 1986). Similarly, in Australia, Facelli and Brock (2000) found that P-rich spots persisted for 50 years after the death of Western myall (Acacia pa-pyrocarpa), but N was lost rapidly from former shrub islands. At the JER, Virginia (unpublished data) observed a degradation of the N pool in shrub islands within 15 years after spraying mesquite with herbicides.


• Recovery of desert shrub vegetation on cleared areas is most rapid when the original soil conditions, such as the islands of fertility, remain intact. Wallace et al. (1980) found more than twice as much shrub biomass regenerated on bare, undisturbed desert soils compared to plowed, disked, or scraped soils after 20 years of plant succession in the Mojave Desert.


• Water, wind, and animals transport nutrients, creating strong spatial patterns of resources


• N inputs in the JER are relatively small compared to those in other deserts.


• Outputs of N are relatively large, especially via wind and water erosion from shrublands. Leaching below the root zone is infrequent and much less important than gaseous losses. Plant uptake of nutrients is enhanced by mycorrhizae and fungal endophytes and controlled by temperature and water availability. Decomposition is rapid, but not strongly related to water availability or C/N ratio of the substrate. Termites are especially important in speeding the decomposition of surface litter and some photo-oxidation also occurs. Decom-position of subsurface litter is mainly by fungi and bacteria, mediated by mi-croathropods, and are more strongly influenced by soil moisture.


• In grasslands, N cycling is relatively faster with lower outputs


• Restoration or remediation of landscapes in the Chihuahuan Desert (Chapter 14) could be improved by understanding, accommodating, and possibly managing the redistribution of soil resources.


• 07 Biogeochemical Fluxes across Piedmont Slopes of the Jornada Basin


• inselberg (i.e., isolated mountain)


• Lehmann lovegrass (Eragrostis lehmanniana), an exotic introduced to control erosion along the power line that crosses the bajada.


• [physical and biological crusts (cyanobacteria) form on the surface of the land and can increase runoff/decrease infiltration]


• [animals dig small holes (10-200mm) into the crusts looking for food, the next storm fills these]


• Where there is a significant proportion of gravel in the surface soil, a gravel lag accumulates in intershrub areas and impedes erosion in general and rill formation in particular.


• [rill flow decreases with distance from head]


• the movement of water, sediment, and nutrients begins in these [inter-rill] areas


• rain-drops falling at terminal velocity are capable of splashing particles with diameters up to 12 mm in all directions (Kotarba 1980).


• The rate of splash transport decreases as overland flow becomes deeper, and it decreases as vegetation, litter, and gravel covers increase.


• [rainfall splash is] relatively minor transport process, accounting for only 5-25% of the sediment transported by overland flow (Abrahams et al. 1994).


• main role of rainfall is to detach or loosen soil particles and lift them into the flow, which then transports them downslope.


• Raindrops not only loosen soil particles but also compact the soil surface and produce a crust (Moore and Singer 1990).


• Shallow overland flow generally does not exert sufficient shear stress on a soil surface to detach soil particles.


• [more sediment moves early in the flow event, dislodged from rain or prior weathering. Sediment transported decreases with time. If animals have been digging (loosening particles) then there is no telling how fast the sediment will flow in a rain event.]


• concentrations of total dissolved N in runoff (total N loss divided by total water loss) were 1.72 mg/L, l.44 mg/L, and 0.55 mg/L for the grassland, shrub, and intershrub plots, respectively. Weighted by the average cover of shrub (38%) and intershrub (62%) areas on the landscape, the mean nitrogen concentration was 0.77 mg/L in the runoff from shrublands.


• runoff from shrub areas is strongly influenced by subcanopy veg-etation, runoff from intershrub areas is largely controlled by surface crust-ing.


• As a result of crusting, runoff from intershrub areas is about three times that from shrub areas, and runoff from degraded grassland is about three times that from grassland.


• Runoff coefficients for the grassland and shrubland are 6% and 19%


• in all three cover types, more than half the N transported in runoff is carried in dissolved organic compounds. Analyses of the data for natural rainfall events indicate that average nutrient losses from the shrub-land is 0.33 kg/ha/yr, which is more than twice the value of 0.15 kg/ha/yr obtained for the grassland. Moreover, these data confirm that the greater nutrient losses from the shrubland are due to higher runoff rather than higher nutrient concentrations in runoff.


• 08 Water and Energy Balances within the Jornada Basin


• Energy Balance Characteristics of the Jornada Basin 08


• net energy balance of the land surface is determined by (1) inputs (radiant energy), (2) outputs (reflection [i.e., albedo], emission of longwave radiation, con-vective heat transfer to the atmosphere [i.e., sensible heat flux], evapotranspiration of water [i.e., latent heat flux], and conduction of heat into soil), and (3) changes in heat storage.


• Jornada Basin is characterized by low mean cloud cover, moderately high elevation (about 1,350 m), moderately low aerosol burden, and low latitude (32.5#N), such that solar energy input is large


• rather re-flective soils (about 35% of shortwave energy is reflected)


• Cooling by evapo-transpiration is often low, as water is frequently depleted from soil surfaces and from vegetation


• clear skies and low water vapor content, which produce low absorptivity for thermal infrared radiation leaving the surface, allow strong radiative cooling. Thus daily fluctuations in surface and air temperatures are very large, commonly exceeding 20#C even without passage of weather fronts.


• photosynthetically active radiation (PAR) band from 400 to 700 nm wavelength, which reduces the landscape's reflectivity (i.e., albedo)


• Changes in the composition of vegetation and the amount of vegetation change the albedo of a landscape. For example, shrub encroachment into former grasslands generally results in higher albedo, because bare soil usually increases, and it is generally lighter in color and thus more reflective (Kurc and Small 2004).


• Another energy loss is conduction of heat into soil (QS or G). Hot soil surfaces send heat into soil.


• Soil heat flux is very modest in thicker vegetation, such as crops, because soil is shielded from much radiant exchange


• variable spatially depending on whether soil heat flux is being measured in open interspaces, partial canopy cover, or full canopy cover (Kustas et al. 2000).


• surface soil temperature in the summer may range between 20C at night and 62C in the day. Thus the mean temperature in the top 10 cm is close to 41C, and below 10 cm in depth soil temperature begins to damp with depth to eventually reach the long-term mean of 15C.


• falloff is about 1C per 10 cm


• Plants affect the partitioning of latent heat flux into its com-ponent fluxes of evaporation and transpiration


• If ET is curtailed, due to soil water deficits, while solar inputs are not changed, the other losses must increase, with surface temperatures rising to strike the new balance. Indeed, the temperature difference from surface to air is a useful measure of water stress, as well as of sensible heat flux (in crops, Jackson et al. 1981; over arid lands and other sparse vegetation, Humes et al. 1994; Shuttleworth and Gurney 1990; Kustas et al. 1994).


• Water Balance Characteristics of the Jornada Basin 08


• simplified version of the water budget for the region is: Precipitation + Run-in = Evapotranspiration + Runoff + Recharge


• Hydrologic recharge of deep soil is minimal in most years (Phillips 1994)


• Playas, fissures, gullies, and burrow pits have recharge rates up to 120 mm/yr (Scanlon and Goldsmith 1997) but cover a very minimal portion of surface area. Thus, essentially all precipitation is lost through soil evaporation (E) and plant transpiration. Despite the lack of water inputs, salinity is rare in soils of the Jornada Basin. However, calcium concentra-tions can exceed limits tolerated by calcifuge vegetation.


• Ten of the 11 shrub species had roots that penetrated the petrocalcic and calcic horizons and grew to depths of 5 m (roots were not mapped below this depth). Roots of 11 grass species were found to extend radially between 0.5 m and 1.4 m in sandy soils but did not penetrate petrocalcic and calcic horizons and did not extend to depths greater than 1.6 m. The conversion from grassland to shrubland may affect the water balance of the Jornada Basin because deep-rooted shrub species may transpire deep water to the atmosphere that was previously below the root zone of grass and forb species.


• many shrub species, particularly the ev-ergreen creosotebush, have longer periods of phenological activity than grasses, which extend the period of time that water is transpired to the atmosphere (Reyn-olds et al. 1999b).


• The partitioning of ET depends on rainfall characteristics, plant spe-cies composition, plant phenology, and photosynthetic pathway of vegetation cover, soil and air temperatures, and nutrient availability.


• Tromble (1983) found that 20% of artificially applied rainfall was intercepted by creosotebush leaves. If an average storm size of 3 mm is used, nearly 12% of 3 mm rainfall may be lost through this pathway in creosote-bush communities (Tromble 1983).


• vegetation change on the Jornada may have little effect on regional precip-itation because total accumulated ET is generally low and the recycling ratio (fraction of precipitation derived from local evapotranspiration; Eltahir and Bras 1996) is low. The recycling ratio clearly increases with spatial scale (becoming 1.0 over the globe) and is highest for any choice of spatial scale in Amazonia.


• it is unclear if these small changes can produce changes in weather pat-terns.


• On larger scales, the feedback of depleted water supply to depleted atmo-spheric humidity to depleted rainfall is expected (Bravar and Kavvas 1991). Con-sequently a drought cycle must be broken by a large-scale disturbance in atmospheric circulation.


• Measurement of Energy Balance and CO2 and H2O fluxes in the Jornada Basin 08


• The proportion of soil E to total ET ranged from 0.3 to 0.6, increasing in creosotebush and mesquite communities probably because of large open interspaces between shrubs. The ratios are biased toward high soil E rates because the mini-lysimeters were measured following precipitation events.


• indicating that shallow soil water is the primary source of water for ET losses and direct evaporation is likely a large component of ET


• Soil heat fluxes at the shrubland site at midday were 30% greater than the grassland, probably due to the 30% greater amount of bare ground in the shrub community.


• Concerns over global climate change have lead to regional networks to measure net ecosystem exchange of carbon (NEE). Though much research has been done in forested systems, far less data exists in semiarid and arid rangelands.


• These data indicate that for the majority of days in a year, the Jornada Basin hovers around zero, or there is a slight source of carbon efflux to the atmosphere. [fig pdf 204]


• When rainfall events occur, there is an initial burst of carbon efflux, which is likely due to increased heterotrophic activity of soil microbes. However, if the rainfall is of large magnitude, the system may shift to net carbon uptake due to increased photosynthetic assimilation of plants and perhaps assimilation by bio-logical soil crusts (Cable and Huxman 2004). The analysis by Mielnick et al. (2004) found that when averaged through time the Jornada Basin was a fairly significant source of carbon efflux through this time period (+145.3 g C/m 2 /yr).


• [this was only looked at on grasslands, and 57% of the days'' data showed zero flux in CO2. The small amount of release of CO2 was multiplied by a longer time period to get such a high source result. Wet or dry years have an affect. 0.07-1.8 g/m2/d of CO2 release are estimates. Driven by pulse rains.]


• <P>Daily ET rates were at a maximum in July through September, and maximum ET rates ranged from 5 mm/day in dry or average precipitation years, but increased to 7 mm/day in the two wet years. Average ET rates ranged from 0.15 mm/day in December to 2.15 mm/day in August. Average yearly ET was 299 mm, indi-cating that as expected all precipitation was lost to the atmosphere via this path-way.


• Interactions of Plants with the Energy and Water Balance 08


• high incoming solar radiation, high variability in surface and soil temperature, and frequently low water availability, which also limits nitrogen availability


• Drought stress originates from both low soil water content and from high evaporative demand (i.e., atmospheric drought; Schulze 1986). Both types of drought stress are prom-inent in the Jornada Basin.


• four basic strategies: drought escape, drought avoidance, drought tolerance, and drought endurance (Gutschick 1987)


• complete their life cycles outside the times of drought, therefore achieving drought escape


• Drought tolerance is the abil-ity to tolerate low tissue water status and maintain continued physiological func-tion


• drought avoidance, some plants, largely perennials, retain access to water supplies, by being phreatophytic (Neilson 1986) or by accessing deep water, and partially decoupling physiologic function from highly variable summer precipitation


• Drought tolerance is the abil-ity to tolerate low tissue water status and maintain continued physiological func-tion. Common drought tolerance mechanisms are osmotic adjustment to maintain cell volume (and turgor in plants) and accumulation of solutes that protect mem-branes, (Jones 1992 discusses these mechanisms in plants)


• drought endurance, ceasing function to help maintain viability in drought. Cryp-togamic cyanobacteria in soil crusts are an extreme example in that they tolerate drying to perhaps -100 MPa in water potential and temperatures up to 70C.


• Jornada plants reduce the tempera-ture load by having small leaves that have effective leaf-air heat transfer, so that even leaves in full sun with minimal transpiration do not rise far above air tem-perature.


• tarbush has the largest leaves with a minor axis of about 2 cm, and these leaves are mostly drought-deciduous


• C4 plants possess an effective bio-chemical pump for concentrating CO2. Consequently they can maintain higher intracellular concentrations of CO2 (ci) at lower leaf conductances and smaller stomatal apertures. Reduced stomatal aperture reduces the amount of water lost to atmosphere in exchange for gaining CO2. Therefore the efficiency of water use, the ratio of carbon gain per unit water loss, is generally higher for C4 species relative to C3 species.


• The temperature optimum and light saturation point is also higher for the C4 plants than for C3 plants. These characteristics of C3plants may make them more adapted to dry environments, but as CO2 levels increase, the relative advantage may become less important. Also, conservative use of water may not be the best strategy if competing species extract water at high rates.


• Empirical evidence exists, in a variety of systems, that drought fitness (DF) and water-use efficiency (WUE)3 are negatively correlated (Thomas 1986; Grieu et al. 1988).


• A more mechanistic relation may be afforded by the observations for creosotebush. Continued respiration for photodamage control or repair requires continued assimilation and transpiration. This reduces season total WUE by re-quiring water expenditure at times of inherently low WUE (high vapor pressure deficit). Continuous recovery processes in drought-active plants may enable plants to quickly use evanescent water resources when they become available.


• Conclusions 08


• The Jornada Basin, like other arid regions, is a net source of sensible heat flux to the atmosphere, a weak source of latent heat flux, and a modest sink for net radiation.


• Evapotranspiration is appoximately 95% of total precipitation inputs.


• The C4 grasses are primarily active during the warmest months and, being shallow-rooted, rely principally on summer rains for their survival.


• The C3 shrubs are remarkably resis-tant to a paucity of summer rainfall.


• These flux data support a conclusion that these arid grasslands are a potential carbon source. However, CO2 fluxes in the basin are strongly driven by large rainfalls, and a longer record of CO2 flux in different vegetation communities is required before we understand these carbon dynamics.


• 09 Eolian Processes on the Jornada Basin


• In the Jornada Basin, wind erosion is the only significant mechanism for the net loss of soil materials because fluvial processes do not remove materials from the basin.


• Airborne dust has a significant residence time in the atmosphere and acts to modify the radiative properties of the atmosphere, mainly by back-scattering the incoming solar radiation (Andreae 1996). Changing land uses in arid and semiarid areas (e.g., overgrazing and cultivation) can drastically alter the dust emissions (Tegen et al. 1996)


• Atmospheric dust decreased the total ra-diative balance of the underlying surface and at the same time induced general warming of the underlying surface-atmosphere system due to a decrease in the system albedo over the arid zones.


• wind erosion involves nonlinear and threshold processes


• Owen Effect 09


• The Owen effect is a feedback mechanism by which airborne sediment increases the drag coefficient of the surface. Before sand-sized particles are injected into the air (saltation), the air contains momentum that is transported by turbulent eddy transfer. During saltation, the sand grains interact with the air and transfer a part of the wind momentum to the ground. This occurs because the saltating particles strike the ground in a parabolic trajectory (implicit in the definition of saltation).


• Soil Moisture Effect on Threshold Friction Velocity 09


• Soil moisture can increase the threshold friction velocity of a soil (Chepil 1956; Bisal and Hsieh 1966; Saleh and Fryrear 1995). McKenna-Neuman and Nickling (1994) showed that sand grains are held together by the capillary effect of soil moisture.


• Aggregation or crust formation usually occurs with the drying of a moistened surface.


• De-struction of the crust and surface aggregates depends on sandblasting, freeze-thaw cycles, formation of crystals, and temperature of the sediment.


• Effect of Vegetation on Wind Erosion at the Jornada 09


• five vegetation categories having three sites each. The veg-etation categories are black grama (Bouteloua eriopoda) grassland, creosotebush (Larrea tridentata), tarbush (Flourensia cernua), playa grassland (characterized byPleuraphis mutica), and mesquite (Prosopis glandulosa). Results for January 30 through April 30, 1998 (a period of active wind erosion), are summarized in table 9-2.


• Of the five vegetation groups at the Jornada, only the mesquite group is a large-scale dust emitter. The mesquite sites contributed dust at a rate roughly a tenth as large as that from a disturbed bare soil site.


• Vegetation acts as a noneroding roughness element that affects the erosion threshold in two ways: (1) directly covering part of the surface and thus protecting it, and (2) absorbing part of the wind momentum that is not then available to initiate particle motion.


• In short, the Jornada possesses a kind of vegetation (mesquite-dominated) that is a relatively poor protector of the soil. ,,, For the other kinds of veg-etation at the Jornada ANPP sites (see chapter 11), however, the theory seems to agree with observations that protection of the soils is adequate.


• Historical Deposition Rates of Eolian Material 09


• In 1962, seven dust traps consisting of 30- by 30- by 5-cm pans filled flush with 1-cm diameter glass marbles were placed at a height of 90 cm (Gile and Grossman 1979). They were placed in the field each year for 11 years from February through June (the dusty season of the year).


• Using an average bulk density of 1.4 g/cm3 soil would accumulate at a rate of 2.4 cm per 1,000 years. Thirty-four percent of the deposited material is silt-sized and 24% is clay-sized material.


• Mean rates of net soil loss for 1933-80 were established by measuring the change of level on grid and transect stakes. The mean rate of soil loss per year for the deflated areas at the site was 5,200 g/m2/yr, whereas the mean rate of net soil loss (for the entire area) for the same site was 1,400 g/m2/yr. Hennessy et al. (1986) determined that there was almost no net loss of sand from the site as a whole; silts (84%) and clays (16%) accounted for all the soil lost.


• This rate of deposition is much smaller than the mean source rate at the natural revegetation exclosure site of 1,400 g/m2/yr.


• rate of emission is 73 times the rate of deposition, the Jornada is almost certainly a source area for dust.


• Erosion Rates Measured on Vegetation-Free, Sandy Soil Locations 09


• mean annual lowering of the soil surface by 1.97, 2.9, and 3.3 cm/yr


• annual mean ver-tical loss of silt and clay particles. These loss estimates are 6,140, 6,904, and, 6,106 g/m2yr


• These emissions are larger than the annual mean net emission rate of 1,400 g/m2/yr reported by Gibbens et al. (1983) above; however, the instrumented site is bare, and the sites evaluated by Gibbens and Beck (1988) were protected by mesquite plants.


• loss of silt and clay from the vegetation-free site is about 5-10 times higher than the loss of silt and clay from the mesquite areas


• Wind is a major geomorphic force in the Chihuahuan Desert and is responsible for many of the soil patterns in sandy areas (Gile 1966a)


• Simonson (1959), there are four soil-forming processes that transform material at the Earth’s surface. These are losses, additions, transloca-tions, and transformations.


• the A horizon (i.e., topsoil) of any soil contains much of the humified organic matter, N, available P, microbial population, and the seed bank, the erosion of this layer results in a loss and redistribution of these biotic components (e.g., Schlesinger et al. 1990)


• because water-holding capacity is largely controlled by organic matter and silt content (Herbel et al. 1994), removal of these constituents will decrease a soil's ability to retain water.


• Organic matter is also important for aggregate formation (Brady and Weil 1996). Its decline therefore leads to a decline in infiltration, which in turn leads to greater runoff (Bull 1991)


• Microporosity causes water to be held more tenaciously than water is held in soil material with larger pores (Hennessy et al. 1983b). Consequently, calcic and petrocalcic ho-rizons are important for preserving sources of water below the layers during droughts for both grasses (Herbel et al. 1972) and shrubs (Cunningham and Burk 1973).


• Because of increased bare ground, additions to the soil profile, in the form of dry dust, wet dust, ions in rain, and organic matter, would not be retained as readily as on noneroded soils covered with vegetation.


• Losses in the depositional setting are mainly derived from silt winnowed from sand (Hennessy et al. 1986)


• In some cases, 86.9 cm of sandy sediments have accumulated in 45 years (Gibbens et al. 1983)


• Conclusions 09


• undisturbed sandy soils populated by mesquite and disturbed soils of all types would be expected to be erodible. Other soils are erodible, but only at very high winds


• most vulnerable soils are the sandy soils - silt soils, playas, and gravelly soils are less vulnerable - clay soils are less vulnerable than sand soils but more vulnerable than silt/playa/gravel


• Change of vegetation cover leads to a change of the source/sink relationship. Grass is one of the most effective protectors of the soil with respect to wind erosion. When grass is replaced by mesquite plants surrounded by large areas of bare soil, wind erosion increases dramatically. Wind erosion also increases dramatically when grass cover is reduced by drought or distur-bances. Vegetation patterns are very important determinants of wind erosion on all soil types.


• highly likely that the Jornada Basin is a net source of eolian materials


• Historical studies show that soil loss rates for mesquite areas having sandy soils (1,400 g/m2/yr) are much greater than the average dust deposition of 19.4 g/m2/yr.


• 10 Plant Communities in the Jornada Basin: The Dynamic Landscape


• Plant communities of the Jornada Basin are characteristic of the north-ern Chihuahuan Desert both in structure and dynamics


• Although a number of plant communities can be differentiated, five major vegetation types are often distinguished: black grama (Bouteloua eriopoda) grasslands, playa grasslands, tarbush (Flourensia cernua) shrublands, creosotebush (Larrea tridentata) shrub-lands, and mesquite (Prosopis grandulosa) shrublands


• major shifts in vegetation composition over the past 50-150 years (York and Dick-Peddie 1969)


• shift in life form due to woody plant encroachment into perennial grasslands (Grover and Musick 1990; Bahre and Shelton 1993).


• A number of drivers have been implicated in these grass-shrub dynamics, including various combinations of livestock grazing, small animal ac-tivity, drought, changes in fire regime, and changes in climate (Humphrey 1958; Archer 1989; Allred 1996; Reynolds et al. 1997; Van Auken 2000). The causes of shrub invasion are quite variable and often poorly understood, although the consequences consistently lead to the process of desertification (Schlesinger et al. 1990).


• Vegetation Characteristics 10


• Vegetation in the Chihuahuan Desert region has been classified as desert-grassland transition (Shreve 1917), desert savanna (Shantz and Zon 1924), desert plains grasslands (Clements 1920), desert shrub grassland (Darrow 1944), and shrub-steppe (Kuchler 1964).


• 77 families, 285 genera, and 490 species have been identified from the region (Allred 2003). Predominant families include the Aster-aceae (59 genera, 93 species), Poaceae (35 genera, 78 species), and Fabaceae (18 genera, 36 species). A complete species list for the Jornada Basin can be found online at http://jornada-www.nmsu.edu.


• Each vegetation type is often associated with a particular soil type and geomorphic position; ecotones between vegetation types in time and space can also be found (Wondzell et al. 1996). A variety of winter and summer annuals are seasonally abundant in all communities (nomenclature follows Allred 2003).


• Black Grama Grasslands 10


• dominant grass species on sandy or gravelly upland


• dominance is highest on deep, loamy soils (Paulsen and Ares 1962).


• weak calcium carbonate layer is often present below the surface


• On sandier soils, dropseeds (Sporobolusspp.) and threeawns (Aristidaspp.) as well as forbs are more abundant. In wet years, black grama cover can reach up to 75% of total vegetative cover, whereas in dry years, black grama can average 44% of the total (Paulsen and Ares 1962). Black grama is an important forage species year round (Wright and Streetment 1958; Paulsen and Ares 1962). This long-lived (35-40 years) C4 grass


• low and infrequent production of viable seeds


• Vegetative spread occurs through the production of stolons (Nelson 1934).


• Associated species in black grama grasslands include mesa dropseed (Sporo-bolus flexuosus), purple threeawn (Aristida purpureaNutt.), and spike dropseed (S. contractus). These species often dominate where black grama has been elim-inated.


• Associated shrubs include soaptree yucca (Yucca elata), broom snakeweed (Gu-tierrezia sarothrae), ephedra (Ephedra trifurca), and scattered mesquite.


• Playa Grasslands 10


• found in low-lying areas with heavy, clayey soils and run-in water are typically dominated by tobosa (Pleuraphis mutica), side oats grama (Boute-loua curtipendula), and alkali sacaton (Sporobolus airoides). Soils are often im-pervious to deep water infiltration with slow infiltration rates


• cemented calcium carbonate layer is rarely present. Up to 80% of total cover in wet years and 62% in dry years can be attributed to tobosa on these sites (Paulsen and Ares 1962). Tobosa is a highly productive, drought-tolerant C4 grass that is palatable only during the growing season (Paulsen and Ares 1962). This short-lived species (7 years) - expanding vegetatively through the production of rhizomes (Neuenschwander et al. 1975; Wright and Van Dyne 1976; Gibbens and Beck 1987).


• Burrograss (Schleropogon brevifolius) and ear muhly (Muhlenbergia arenacea) are also found on heavy soils that may or may not receive run-in water. can compose up to 60% of total cover - ability to begin growth under cool temperatures and to spread rapidly by stolons and seeds.


• Tarbush Shrublands 10


• on clay loam soils with some gravel near the surface (Paulsen and Ares 1962). These sites may receive some run-in water. Tarbush is a deciduous C3 perennial shrub with a tar-like odor as a result of secondary compounds in its leaves (Estell et al. 1998). Tarbush has an extensive root system and produces seeds that are wind or water dispersed (Mau-champ et al. 1993; Gibbens and Lenz 2001). Herbaceous species in these com-munities are similar to the playa grasslands, where tobosa and burrograss are the common grasses.


• Creosotebush Shrublands 10


• bajada slopes and alluvial fans. Soils vary from well-drained sands to shallow stony soils underlain by cemented calcium carbonate at shallow or deep depths (Gardner 1951). Creosote makes up 28-45% of total cover in these communities (Paulsen and Ares 1962). Creosote is an evergreen, drought-resistant C3perennial shrub that can live up to 400 years in the Chihuahuan Desert (Miller and Huenneke 2000). Creosotebush produces many secondary compounds that may influence the growth and survival of other species (Knipe and Herbel 1966). Associated species in these communities include bush muhly (Muhlenbergia porteri), fluff grass (Dasyochloa pulchella), and black grama, as well as a variety of forbs.


• Mesquite Shrublands 10


• "moving dune complex" (Campbell 1929). Mesquite is a deciduous, thorny, long-lived (200 years) C3 shrub. This species is a facultative phreatophyte with very deep and laterally extensive root systems (Gibbens and Lenz 2001). Mesquite occurs on most soil types but is particularly prevalent on sandy soils. Most mesquitedominated soils are deep sands with a calcium carbonate layer at variable depths. Short, multistemmed mesquite plants accumulate blowing sand until a mound forms around each plant. Interspaces between plants are scoured of loose soil resulting in sparse herbaceous cover. Mesquite typically makes up 30-55% of total cover on these sites (Paulsen and Ares 1962). Associated plants in dunes include saltbush (Atriplex canescens), broom snakeweed, dropseeds, threeawns, and a variety of forbs.


• Vegetation Dynamics 10


• Collectively, mesquite, tarbush, and creosotebush occurred on 42% of the Jornada Experimental Range (JER) in 1858, and no area was free of shrubs by 1963. classic paper by Buffington and Herbel (1965)


• Early land survey notes also showed that the increase of shrubs was widespread in southern New Mexico (York and Dick-Peddie 1969)


• One of the striking features of vegetation change from 1915-16 to 1998 is the large reduction in black grama-dominated grasslands (figures 10-1, 10-2)


• black grama was the first or second dominant on 28% of the Jornada in 1915-16, this species dominated only 4% of the area in 1998 for a net loss of 24% (table 10-1)


• Threeawns were much more prominent in 1915-16 than in 1998 (figures 10-1, 10-2), mainly on areas now dominated by mesquite.


• reduction in dominance was generally due to encroaching shrubs and associated effects of aeolian deposits (Gibbens and Beck 1987). Several minor grass species showed no change in area dominated, including fluff grass, sand muhly (Muhlenbergia arenicola), and vine mesquite (Panicum obtusum). Bush muhly and the introduced Lehmann lovegrass (Eragrostis lehmanniana), on the other hand, have expanded.


• Similar to the dropseeds, broom snakeweed is relatively short-lived and populations fluctuate widely among years, often in response to winter precipitation.


• sand sage (Artemisia filifolia)


• Drivers of Vegetation Change 10


• widespread expansion of two shrubs (honey mesquite and creosotebush) into perennial grasslands has been particularly important. Much of this expansion has been accompanied by a decrease in cover and abundance of black grama, a key forage species that previously dominated many of the sandy loam uplands in the Chihuahuan Desert.


• Livestock density in southern New Mexico increased markedly in the 1880s following the American Civil War (Fredrickson et al. 1998)


• Although the peak in animal numbers in the late 1800s and early 1900s corresponds to a period of rapid invasion by shrubs in the region, other factors interacting with the effects of livestock likely contributed to the invasion process. A number of studies have examined the response of vegetation following livestock removal. In many cases (but not all) perennial grasses increased following exclusion of livestock.


• Livestock have four major effects on vegetation that differ for grasses and shrubs: (1) herbivory on aboveground biomass (for grasses: tillers, inflorescences, seeds; for shrubs: leaves, stems, seeds), (2) dispersal of seed, (3) trampling of plants and soil, and (4) redistribution of nutrients, particularly nitrogen, through urine and fecal deposition.


• Basal area and biomass of black grama can decrease as much as 50% in three to four years when utilization is greater than 40% under seasonal or year round grazing (Canfield 1939; Valentine 1970). In the absence of cattle or under light to moderate grazing (less than 40% utilization), black grama is primarily influenced by precipitation (Paulsen and Ares 1962)


• Regrowth of this species [black grama] was negligible after 13 years of cattle exclusion (Nelson 1934; Canfield 1939).


• grazing also has direct positive effects on shrubs - particularly honey mesquite recruitment


• Cattle readily consume mesquite pods, and large numbers of seeds remain viable after passing through the digestive tracts of cattle (Paulsen and Ares 1962; Mooney et al. 1977). Seeds are often deposited at large distances from where they were consumed (Humphrey 1958; Paulsen and Ares 1962). A favorable microenvironment provided by cattle dung may facilitate germination, particularly if the dung is deposited in the rainy season (Paulsen and Ares 1962). Seedlings of honey mesquite readily become established; within four months following germination, seedlings develop roots to depths of 40 cm (Brown and Archer 1990).


• small animals primarily affect plants through their consumption of aboveground material and, in particular, seeds and seedlings.


• When perennial grasses are abundant, small animals appear to have minimal impact on grassland dynamics (Norris 1950; Buffington and Herbel 1965; Gosz and Gosz 1996). However, in open areas following shrub invasion, jackrabbits (Lepus californicus) and rodents are generally more abundant (Vorhies and Taylor 1933), and small animal activity in overgrazed areas can prevent a recovery in perennial grasses (Norris 1950).


• Granivory by ants may be more important than loss of seeds to rodents for various plant species in black grama grasslands (Kerley and Whitford 2000).


• Dropseeds are preferred forage for jackrabbits - up to 40% of their diet (Dabo et al. 1982)


• Positive effects of lagomorph exclusion on black grama were expected because this species can comprise up to 54% of the diet of jack-rabbits in summer months (Fatehi 1986; Fatehi et al. 1988)


• Black grama did not show a response until 51 years of lagomorph exclusion in areas where shrubs had been removed (Havstad et al. 1999).


• diet of jackrabbits is comprised of honey mesquite, creosotebush, and broom snakeweed stems that are used as sources of food and water (Dabo et al. 1982; Steinberger and Whitford 1983b; Fatehi 1986; Fatehi et al. 1988).


• Canopy cover of honey mesquite and creosotebush were significantly higher on lagomorph exclusion plots within 10 years after the start of the treatments and persisted for greater than 40 years (Gibbens et al. 1993; Havstad et al. 1999).


• Rodents can aid in the dispersal of honey mesquite


• Merriam kangaroo rats (Dipodomys merriami) collect seeds and store them in shallow caches; many of these seeds are not consumed and can germinate to contribute to mesquite expansion (Paulsen and Ares 1962).


• Drought 10


• Severe drought (Palmer drought index between -3 and -4) of 2-4 years - duration occurs on average every 20-25 years, whereas extreme drought (palmer index less than -4) occurs every 50-60 years (Scurlock 1998).


• drought is a contributing factor in shrub invasion (Herbel et al. 1972).


• In general, basal area of black grama and other perennial grasses decrease during drought with black grama recovering more slowly than other grasses.


• Grazing intensity was found to have little effect on black grama basal area during extended drought, although recovery was faster for conservatively grazed plots compared with heavy grazing or protected plots (Paulsen and Ares 1962).


• most dramatic change in vegetation occurred during 1950-61 when black grama disappeared from an additional 37 quadrats. [104 total, 11 already extinct]


• Tobosa grasslands are typically located in low-lying areas with heavy soils that receive some run-in of water from overland flow (Herbel and Gile 1973) that may explain the fast recovery


• Burrograss in run-in locations showed similar responses to tobosa


• threeawns as well as dropseeds were more susceptible to drought compared with black grama; the threeawns were eliminated by the drought, whereas the dropseeds made a partial recovery through seedlings (Herbel et al. 1972).


• In general, shrubs are less severely impacted by summer drought compared with grasses.


• long-term studies have not been conducted at a sufficiently large scale.


• Observations through time suggest that shrubs now dominating former black grama sites became established during or following the 1950s drought. An analysis of changes in dominant species showed that creosotebush, mesquite, and tarbush became dominants on former black grama sites by 1981 (Gibbens and Beck 1987).


• Little is known about the role of fire in the Chihuahuan Desert, either in the United States or Mexico (Humphrey 1974; Drewa et al. 2001).


• Chihuahuan Desert grasslands, natural fires most likely occurred in June, when the frequency of lightning strikes is high (Gosz et al. 1995), the vegetation is dry, and weather conditions exist to promote the spread of fire (low humidity, high temperatures, and high winds). A characteristic fire return interval is unknown, but a 9-10-year period has been estimated for desert grasslands in southeastern Arizona (Cable 1967; McPherson 1995).


• Prior to a period of intense livestock grazing and subsequent encroachment of shrubs during the late nineteenth century, fuel loads were likely higher than at present. Additionally, despite reduced fuel loads, natural fires are still observed every two to three years in ungrazed, black grama- dominated grasslands in central New Mexico at the Sevilleta LTER, and an extensive natural fire was observed in the early twentieth century on the Jornada. Thus, though fires continue to occur naturally in Chihuahuan Desert grasslands, it is likely that the size, frequency, and intensity of these fires have decreased over the past 150 years (Bahre 1995).


• In general, effects of fire on perennial grasses are highly variable; grass response may be influenced by soil moisture conditions at the time of the fire, the amount of rainfall received during the growing season immediately following the fire, or the grazing intensity by cattle (Gosz and Gosz 1996).


• Most research conducted on black grama indicates high mortality following fire under drought conditions that persist into the immediate postfire environment.


• suggest that black grama can recover rapidly from even yearround cattle grazing (utilization less than 40%) if growing season precipitation is at least equivalent to the long-term average immediately following a fire (Drewa et al. 2001).


• Grass mortality was a function of the size of individual plants prior to fire. Small plants (basal area less than 10 cm2) had a higher probability of fire-induced mortality than larger plants (>30 cm2)


• Prior to European settlement, natural fire may have been effective in deterring honey mesquite invasion by top-killing or completely killing plants.


• One year following prescribed fires in June 1999, canopy area of honey mesquite decreased by 22% but increased 24% in unburned areas (Drewa et al. 2001). In the same study, shrub volume decreased 40% following fires and increased 30% after just one year in fire-excluded areas. In addition, only 3 small shrubs (less than 65 cm height; 10 years old) of the 210 shrubs were killed completely.


• For shrubs clipped to simulate fire, resprouting was 35% greater than that after actual lowintensity fires (fueled by natural vegetation) and 60% greater than that after actual high-intensity fires (artificial fuel additions).


• Fire may also be effective in limiting the recruitment of mesquite plants into grasslands. Fire can completely kill seeds and seedlings of mesquite (Cox et al. 1993; Brown and Archer 1999). Although complete kill of larger plants is rare


• Less information is known about fire effects on other species of shrubs, including creosotebush or tarbush. Fire may be important in limiting recruitment of these species into grasslands, but the probability of adult mortality may be lower compared with mesquite, which is a more aggressive resprouter.


• Climate Change 10


• The current shift from grass- to shrub-dominated ecosystems has likely occurred at a faster rate (50-100 years) compared with geologic changes over hundreds or thousands of years, and the influence of human activities has been particularly important in these rapid rates of change. However, shifts between grasslands and shrublands over geologic time suggest that the current shrub in-vasion episode may be reversed if climatic conditions once again become more favorable for the recruitment and growth of grasses.


• Woody plants typically possess the C3 photosynthetic pathway that may confer an advantage under elevated CO2 compared with C4 grasses. Also, widespread encroachment of woody species into grasslands began shortly after atmospheric CO2 concentration rose above its preindustrial level of 270-280 ppm.


• Elevated CO2 may have contributed to the general increase in woody plants globally, but local factors are more likely to be important to the rate, pattern, and extent of invasion at a particular site (Archer et al. 1995).


• [in 2 simulations] A directional increase in year-round temperature and increase in summer precipitation resulted in an increase in establishment of black grama at all sites.


• Two periods of precipitation are predicted to be critical for black grama recruitment: Precipitation in June is important for seed germination followed by precipitation in July and August that is needed for seedling establishment (Peters 2000).


• Recovery of black grama is not expected on degraded sites currently dominated by creosotebush where the establishment of black grama has a very low probability of occurring. By contrast, sites that are currently codominated by black grama and creosotebush on soils with a moderate ability to hold water for plants are predicted to shift to black grama dominance.


• Conclusion 10


• Our studies show that a suite of processes were important in transforming the JER from desert grassland to shrubland over the past 50-100 years. Large numbers of cattle consuming perennial grasses and dispersing mesquite seeds combined with severe drought in the early 1900s and again in the 1950s, along with an increase in small animal herbivory on grasses and a reduction in fire control of shrubs, apparently led to the landscapes that we see today.


• changes in the size, timing, and intensity of rainfall events during the growing season may also shift the vegetation with grasses being favored by small, frequent rainfall and shrubs being favored by large rain events with deeper infiltration.


• 11 Patterns of Net Primary Production in Chihuahuan Desert Ecosystems


• USDA Jornada Experimental Range (JER) in 1912 - Chihuahuan Desert Rangeland Research Center (CDRRC) ... long-term data sets


• premises for arid/semi-arid ecosystems: <OL> Plant productivity is low relative to that of other ecosystems (Lieth 1975). NPP is regulated primarily by localized water availability and hence should be correlated closely with precipitation (Le Houerou 1984; Le Houerou et al. 1988). Noy-Meir's (1973) definition of deserts as "water-controlled ecosystems with infrequent, discrete, and largely unpredictable water inputs." A pulse-reserve pattern (Noy-Meir 1973) characterizes the behavior of populations (including producer populations) in deserts, such that the episodic availability of resources in excess of some threshold (such as the "pulse" of precipitation) stimulates growth and the production of a large reserve (e.g., of photosynthetic tissues, propagules, and organic matter). Deserts, especially shrubland systems, are dominated by long-lived, stress tolerant plants with slow growth and low population turnover. Soil texture has been proposed as an important determinant of aboveground NPP in arid and semiarid systems, with coarse sandy soils having greater infiltration of water to depth, lower rates of evaporative loss from the surface, and therefore greater water availability - This has been termed the inverse-texture hypothesis (Noy-Meir 1973; Sala et al. 1988), because the reverse is predicted to occur in more humid regions where water-holding capacity is presumed to be more important than infiltration in determining soil moisture availability. Abiotic constraints and the physiological tolerances of organisms, rather than biological interactions among organisms, dictate productivity levels. In other words, deserts represent areas where plants are stress tolerators, rather than competitors or ruderals. </OL>


• One of the major conclusions from the Jornada IBP work on desert shrublands (Ludwig 1987) was that localized production (in small parts of the landscape) can be extremely high, as table 11-1 suggests.


• Sampling occurs three times per year: February, to capture winter annuals; May, timed to measure shrub leafout and to reflect a late-season sample of the spring bloom of annuals; and September-October, representing the peak biomass for the greatest number of annuals and other fall-flowering species.


• v(figure 11-1; Huenneke et al. 2002 with additional unpublished data more recently available) showed that grasslands exhibit the greatest variation in production values over time. creosotebush-dominated sites demonstrate extremely regular seasonality, with the peak of production nearly always occurring in spring. Mesquite shrublands were less markedly seasonal than creosotebush systems, and demonstrated considerable variability among the three sites. Tarbush-dominated systems were remarkably invariant in aboveground biomass and in ANPP over time despite a substantial grass component.


• shrub-dominated systems are patchier in structure than the more homogenous grasslands and that patchiness is exacerbated by self-reinforcing patterns of increased biotic function within shrub patches compared to decreased biotic function in interplant spaces. (Schlesinger et al. 1990; see also chapter 1)


• At the scale of our monitoring, aboveground biomass was patchier in shrub-dominated systems than in the grasslands.


• Our data (Huenneke et al. 2002) confirm that differences between grassland and shrubland productivity, though detectable over the long term, are not enormous when one considers the entire plant community.


• Production Is Constrained by Water and Nutrient Availability 11


• primary productivity might be at least partially constrained by nitrogen (Ettershank et al. 1978). Ludwig (1987)


• water and nitrogen undoubtedly interact. Gutierrez and Whitford (1987a, b; Gutierrez et al. 1988) observed strong interactions between water and nitrogen amendments for annuals


• Local precipitation inputs may be a poor proxy for water availability due to the importance of runoff and run-in patterns in redistributing water.\


• The conclusion was that each mm of rain produces on average 4 kg/ha/yr (0.4 g/m2). For Jornada Basin upland grasslands, the LTER productivity data are about 248 g/m2 from a long-term average annual precipitation of 245 mm, or about twice the productivity predicted by Le Houerou et al. (1988).


• growth appears to respond to precipitation received in a prior season or a prior year rather than the current season.


• We suspect that different species have very different patterns of response to precipitation, and therefore no simple relationship exists between precipitation received in any one interval and total community aboveground production.


• the tremendous range of variation (spatial and temporal) in ANPP values renders most comparisons with published values from arid ecosystems meaningless


• Jornada Basin data show a clear and consistent pattern of coarser textured soils supporting lower perennial grass yields than finer textured soils


• ANPP appears to be greatest in sites on loamy soils


• Mesquite and creosotebush sites have coarser soils than do grasslands but certainly do not have higher average productivity (Huenneke et al. 2002).


• highest ANPP values anywhere are on the heavy soils of playas and the loamy grassland soils. (However, it is true that ANPP is consistently low on the fine-textured soils of tarbush sites.)


• Chihuahuan Desert communities are less complex structurally than most forest types, though more challenging than more mesic grassland or herbaceous communities.


• [communities range from] extremely simple mesquite sites to diverse grasslands


• Fertilization-induced increases in productivity in the first years of the Jornada LTER program were accompanied by modest decreases in species richness (Gough et al. 2000).


• The single most dominant species in each site contributes the largest proportion of ANPP, and only a small number (one or two) of subdominants add any substantive proportion to total community production (chapter 10).


• However, in the first five years of this removal experiment [removing 1 species from a given community], we observed no strong positive responses, questioning that the removal of any one species group in fact releases any other group from water limitation.


• it seems that primary change has been alteration in the identity of the producers [grass- to shrub-land conversion] and a loss in the capacity to respond to favorable years, rather than an overall directional decrease in production rates.


• The biomass accumulation ratio or mean peak biomass divided by annual productivity (Whittaker 1975) is 0.9 for the playas, which contain primarily shortlived herbaceous vegetation whose aboveground portions are replaced completely in each wet season.


• mesquite sites, so strongly dominated by the woody shrub component, had a biomass accumulation ratio of only 1.8 (suggesting complete turnover of aboveground biomass on average within two years).


• One mesquite site (M-NORT) showed a multiyear directional trend in biomass accumulation with aboveground biomass increasing during the 1990-94 period. The magnitude of the increase was substantial. However, the 1994-95 dry conditions coincided with a sharp decline in biomass to former levels, and in the subsequent years there has been no directional change, merely fluctuation in NPP that is typical of the other sites being studied.


• few formal studies of belowground plant biomass and productivity in Jornada ecosystems


• Belowground biomass (and allocation belowground or root:shoot ratio) is generally assumed to be high in arid and semiarid systems.


• Chew and Chew (1965) found that creosotebush near Portal had a rather low proportion of root biomass. Ludwig (1977) presented data for eight species of woody plants at the Jornada showing root:shoot ratios of mean 0.9; he described these as generalized (not specialized) root systems.


• Gibbens and Lenz (2001) have documented very extensive root systems for a number of Jornada plants, demonstrating impressive proliferation in specific soil volumes and suggesting highly plastic and responsive allocation. Brisson and Reynolds (1994) similarly described patterns of root distribution in creosotebush at the Jornada that suggested intricate and plastic relationships among neighboring individuals.


• Pieper et al. (1983) found root:shoot ratios greater than 1 (1.4-2) for vegetation sampled in the long-term forage production study with higher values in grazed than in ungrazed communities. Grasses constitute much of this belowground biomass.


• belowground biomass was less in grazed areas (due largely to the difference in species composition with fewer grasses and thus fewer shallow fibrous roots).


• Ludwig (1977) reviewed a number of studies and concluded that the often stated generalization that desert shrubs have high root:shoot ratios is too simplistic; values for creosotebush at the Jornada, for example, varied from 0.23 to 2.7. Mean values for eight Jornada species averaged 0.925. If we apply this ratio to our average aboveground biomass values (table 11-3), we would estimate that total plant biomass (above- and belowground) ranges from about 300 g/m2 in the tarbush ecosystem type to more than 500 g/m2 in the grassland and mesquite ecosystems.


• NPP - the most basic measure of ecosystem function


• Conclusion 11


• First, these ecosystems are more complex in time than a simple pulse-reserve model would suggest: There are no simple relationships between precipitation input and system function (not even threshold relationships)


• varying life span and multiple modes of response (from vegetative growth to belowground storage to seedling recruitment) may contribute to the potential for complex multiyear dynamics.


• Tremendous spatial variation in rates of aboveground production within ecosystems of a given type. This variation makes it difficult to extrapolate from point-based measurements to larger, management-relevant scales.


• A significant contribution of our studies of plant production is that total community composition includes significant contributions from species other than perennial grasses. Hence, traditional estimates of ANPP (based on forage alone or on clipping of grasses) are far lower than our more dynamic and comprehensive estimates. In the short term, this makes it frustrating or impossible to compare our estimates with those from previous work or other sites


• 12 Chihuahuan Desert Fauna: Effects on Ecosystem Properties and Processes


• Early studies dealt with animal species that were thought to reduce the amount of primary production that was available to support livestock.


• With the establishment of the International Biological Programme (IBP) in the late 1960s and its premise that ecosystems could be modeled based on energy flow, animal studies were designed to measure energy flow through consumer populations. Those studies yielded estimates of consumption of live plant biomass between 1% and 10% of the annual net primary production (NPP) (Turner and Chew 1981).


• From these studies Chew (1974) concluded that in most ecosystems consumers process only a small fraction of the NPP as live plant material but play important roles in ecosystems as regulators of ecosystem processes rather than energy flow. Chew's hypothesis was then the focus of animal studies in the Jornada Basin for nearly 30 years. Studies of animals as regulators of ecosystem processes led to the expansion of Chew's hypothesis to include the effects of animals on ecosystem properties, such as patchiness.


• studies of animal populations were conducted simultaneously


• bear in mind that the published data on animal populations reflect vegetation and ecosystem conditions that are very different from the conditions in which many Chihuahuan Desert species existed only a century before (see chapter 10).


• most abundant and widespread rodents on the Jornada belong to the family heteromyidae (kangaroo rats [Dipodomysspp.], silky pocket mice [Perognathusspp.], and coarse-haired pocket mice [Chaetodipusspp.]).


• Merriam's kangaroo rat (Dipodomys merriami) is most abundant in the shrub-dominated habitats


• Ord's kangaroo rat (Dipodomys ordii) is most abundant in the grassland habitats


• banner-tailed kangaroo rat (Dipodomys spectabilis), a grassland specialist that plays a keystone role in these ecosystems (Mun and Whitford 1990), is absent in the desertified mesquite (Prosopis glandulosa) coppice dunes and creosotebush (Larrea tridentata) and tarbush (Flourensia cernua) shrublands.


• both the abundance and species richness of rodents were higher in shrub-dominated areas than in desert grassland (Wood 1969; Whitford 1976; Whitford et al. 1978b).


• subdominant species in desert grasslands included grasshopper mice (Onychomysspp.), spotted ground squirrels (Spermophilus spilosoma), and silky pocket mice (P. flavus).


• Dry lake basin grasslands and tobosa (Pleuraphis mutica) grass swales are thought to support cotton rats (Sigmodon hispidus) (Wood 1969), whereas pocket gophers (Thomomys bottae) are limited to the piedmont grassland at the base of Mount Summerford of the Dona Ana Mountains


• vegetation growth form, vegetation cover, landscape position, and soil texture determine the spatial distribution patterns of rodents.


• Black-tailed prairie dogs (Cynomys ludovicianus) occurred in scattered colonies in the basin prior to 1917. During World War I these populations were exterminated by government programs to increase forage area for livestock to promote red meat production during the war period. These populations have not returned (Oakes 2000).


• Mean annual abundance of black-tailed jackrabbits was 36/km2 in mesquite shrublands, 30/km2 in mesquite coppice dunes, and approximately 8/km2 in creosotebush and tarbush shrublands. Mean annual abundance in grassland was 5.7/km2. Desert cottontail abundance variedfrom 1.0 to 7.2/km2 in shrublands but only 0.25/km2 in grasslands.


• Foraging pits serve to trap windblown seeds and plant litter (Steinberger and Whitford 1983a)


• More than half of tagged foraging pits in black grama (Bouteloua eriopoda) grassland produced threeawn (Aristida spp.) seedlings and/or seedlings of globe mallow (Sphaeralcea subhastata) (Jackson and Whitford unpublished data)


• rodent digging activities may accelerate erosion rates when loosened sediment is washed away (Neave and Abrahams 2001, see chapter 7).


• Breeding bird densities in black grama grasslands (9.8 breeding pairs/km2) were considerably lower than in the creosotebush shrublands (28.8 pairs/km2) (Raitt and Pimm 1978).


• Breeding/nesting birds were completely absent from the desert grassland site in a year with below-average growing season rainfall.


• Nests of the most abundant species, black-throated sparrows (Amphispiza bilineata), were primarily in creosotebushes at the margins of small drainage channels (86% of the nests). Nests of verdins (Auriparus flaviceps), the second most abundant species, were predominantly in whitethorn (Acacia constricta) growing along the margins of large and small drainage channels (79% of the nests).


• The nests of other species recorded in this study were located in large, riparian shrubs growing in the channels or margins of large arroyos (cactus wren [Campylorhynchus brunneicapillus], crissal thrasher [Toxostoma dorsale], black-tailed gnatcatcher [Pilioptila melanura], and loggerhead shrike [Lanius ludiovicianus]).


• Other species that were recorded on the study area included scaled quail (Callipepla squamata), mourning dove (Zenaida macroura), and roadrunner (Geococcyx californianus).


• most abundant lizard species on the Summerford watershed was the western whiptail (Cnemidophorus tigris). Densities of this species ranged from 30-50/ha on the playa fringe and creosotebush bajada.


• transients or immigrants from nearby source habitats, including the Chihuahuan spotted whiptail (Cnemidophorus exsanguis), Great Plains skink (Eumeces obsoletus), and lesser earless lizard (Holbrookia maculata).


• Only four species were permanent residents of the creosotebush bajada: western whiptail, checkered whiptail (C. tesselatus), round-tailed horned lizard (Phrynosoma modestum), and long-nosed leopard lizard (Gambelia wislizeni).


• greater earless lizard (Cophosaurus[Holbrookia]texana) was limited to the large arroyo habitats on the bajada.


• side-blotched lizard (Uta stansburiana) was associated with the dense vegetation of the feeder arroyos and the large arroyos.


• mesquite-Mormon tea (Ephedraspp.) area fringing the playa lake basin: western whiptail, checkered whiptail, Texas horned lizard (Phrynosoma cornutum), round-tailed horned lizard, side-blotched lizard, desert spiny lizard (Sceloporus magister), and long-nosed leopard lizard (Whitford and Creusere 1977)


• abundance and diversity of lizards was higher in mesquite coppice dunes than in adjacent grasslands.


• Five species of anurans inhabit several areas of the Jornada Basin around ephemeral lakes. Estimated densities of adult anurans (based on mark and recapture) at a playa lake were: western spadefoot (Scaphiopus hammondi), 238/ha; Plains spadefoot (Scaphiopus bombifrons), 206.3/ha; Couch.


• As soon as the playa lake flooded, species of breeding frogs began to occupy different parts of the playa.


• Juvenile toads move to areas of sandy soil around the margins of the playa, where they burrow into the soil to estivate until the next growing season. Overwintering juvenile toads suffered high mortality (70-80%) during the first winter and 50-58% during the second winter (Creusere and Whitford 1977). Survival of juvenile toads is dependent on the quantity of fat the toads can accumulate prior to burrowing into the soil for winter and on the moisture of the burrow sites (Whitford and Meltzer 1976).


• Juvenile toads that die in their overwinter burrows return nutrients concentrated in their natal ponds to the surrounding area. Based on the conservative estimate of juvenile toads of 18,333/ha, this can be an important mechanism of spatial redistribution of nutrients from a nutrient sink to the surrounding landscape.


• Because of their effects on decomposition, nutrient cycling, water infiltration, and spatial distribution patterns of organic matter, subterranean termites are considered keystone species in the Chihuahuan Desert (Whitford 2000)


• subterranean termites were equally abundant in all desertified and undegraded habitats, except for those areas inundated for periods of a month or more (ephemeral lake basins)


• average live biomass of termites, estimated from numbers of termites removed from bait rolls, on the Desert Biome watershed was 3.6 kg/ha. The ratio of termite biomass to livestock biomass based on average stocking rates was 4.4, indicating the potential importance of termites in energy flow in Jornada ecosystems (Johnson and Whitford 1975).


• Abiotic processes (heat and ultraviolet light) decompose detritus located on the soil surface (MacKay et al. 1987a; Moorhead and Reynolds 1989a) or the detritus is consumed by invertebrate detritivores and decomposed in their guts by the symbiotic microflora and microfauna (Crawford 1988).


• most abundant ground-dwelling arthropods were tenebrionid beetles (Araeoschizus decipiens) and orb spiders (mostly black widow [Lactodectus mactans])


• most abundant taxa in both shrubland and desert grasslands are detritivorous (scarabids, tenebrionids, polyphagids, and gryllacridids). The gut symbionts of these detritivores include bacteria, fungi, protozoans, and nematodes (Crawford 1988).


• stink beetles (Eleodes[Coleoptera: Tenebrionidae]) have been observed feeding on the chaff accumulations around the nest disks of seed-harvesting ants (Whitford 1974). Desert cockroaches (Arenivagaspp.) feed on decaying leaves and roots of desert shrubs (Hawke and Farley 1973).


• detritivores are the most abundant Chihuahuan Desert animals and account for the highest biomass of primary consumers (Ludwig and Whitford 1981).


• Although there is overlap in the distribution of the two widespread species (rough harvester ant [Pogonomyrmex rugosus] and desert harvester ant [Pogonomyrmex desertorum]), P. rugosusis absent from mobile and stabilized sand dune areas, and P. desertorumis absent from the clay and silt soils of the tobosa grass swales.


• In a broom dalea (Dalea scoparia) sand dune area, Maricopa harvester ant (Po-gonomyrmex maricopa) replaces rough harvester ant as the large harvester ant in the system.


• The large nests of these harvester ants in turn contribute to patchiness in soil properties due to the to the concentration of nutrients to nests, bioturbation, and chemical alterations of soil via the vertical redistribution of calcium carbonate (Whitford and DiMarco 1995; Wagner et al. 1997; Whitford 2002).


• Soil texture appears to be the most important factor limiting the distribution of most of the other ant species in Chihuahuan Desert ant communities (Whitford et al. 1999; Bestelmeyer and Wiens 2001a).


• attine ant species (Trachymyrmex smithii neomexicanus) that is absent in grasslands occurs in high abundance in adjacent mesquite coppice dunes and has also been recorded in creosotebush communities on deep sandy soils (Wisdom and Whitford 1981). The attine ant species collect senesced leaves and senesced floral parts (petals and sepals) (Schumacher and Whitford 1975). The leaves and floral parts are broken down by fungi cultured by the ants in fungal gardens (Gamboa 1975).


• The earliest studies of microarthropods found that their densities were directly correlated with the amount of surface litter (Santos et al. 1978).


• There were 18 orders of soil microarthropods recorded from 12 sites on the Summerford watershed (Wallwork et al. 1985).


• The initial stage of decomposition of belowground litter is primarily via soil bacteria. The bacteria are grazed by protozoans, primarily naked amoebae, and by bacteriophagous nematodes. The numbers of protozoans and nematodes are regulated by several species of omnivorous microarthropods (Acarina) that prey on the nematodes (Santos et al. 1981).


• When microarthropods were eliminated from buried litter by a broad-spectrum insecticide, bacteriophagus nematode numbers increased dramatically. The large numbers of nematodes overgrazed the bacteria, thus reducing the rate of decomposition.


• In mesic ecosystems, microarthropods affect decomposition and mineralization processes by masticating the litter and passing it through their guts. This increases the surface area and inoculates the litter with microflora from the gut of the arthropods.


• in arid and semiarid regions, the role of microarthropods as regulators of the rate of decomposition is indirect via predation on nematodes and/or fungi, rather than directly by consumption of dead plant material.


• The later stages of decomposition and mineralization in dry soils are regulated by some of the same species of omnivorous mites feeding on fungi. Fungi replace bacteria as the primary microfloral decomposers in dry soils.


• Experiments in which microarthropods were removed showed that rates of nitrogen mineralization were significantly reduced compared with the rates measured when microarthropods were present (Parker et al. 1984b). Elimination of fungivorous and omnivorous mites resulted in a large increase in fungal biomass. Mineral nitrogen from soil surrounding dead roots or buried litter is incorporated into fungal biomass. The fungi use the carbon in litter or roots as energy sources but scavenge nitrogen from the surrounding soil to produce fungal biomass.


• The nitrogen incorporated into fungal biomass is considered immobilized, that is, not available to be absorbed by plant roots. Soil microarthropods that graze on fungal hyphae release immobilized nitrogen as mineral nitrogen in the form of excretory products. These experiments demonstrate that mineralization of nitrogen in desert ecosystems requires the activities of soil microarthropods.


• most abundant insects on shrubs in deserts are sucking insects (HomopteraandHemiptera) (Lightfoot and Whitford 1987; Schowalter 1996; Schowalter et al. 1999).


• The frass and honeydew production from these insects fertilizes the litter under the shrub canopies with soluble carbohydrates and nitrogen. This readily available form of carbon and nitrogen stimulates the growth of microflora on the litter (Lightfoot and Whitford 1987).


• Rapid growth of soil microflora as a result of inputs of high-carbon, low-nitrogen sub-strates results in the immobilization of soil nitrogen in the rapidly growing microbial biomass (Parker et al. 1984b). Nitrogen immobilization imposes severe nitrogen limitations on the biomass production of shrubs and of ephemeral and perennial herbaceous species.


• Pulse-Reserve and Source-Sink Models 12


• In deserts, where many animal species live close to their limit of physiological tolerance for one or more abiotic factors, the responses of species populations to fluctuations in the abiotic environment must be understood before questions concerning the role of animals in ecosystems can be addressed.


• [pulse-reserve is old standard thinking where rains lead to increased production and increased reserves of in fat, honey, desiccation-resistant eggs, etc.]


• Whereas the pulse-reserve model addresses temporal heterogeneity, the source-sink conceptual model addresses the consequences of spatial variation in the transfer of individuals (and resources) between elements of landscape mosaics. This model considers landscapes to be composed of three types of habitats: (1) source habitat, in which reproduction exceeds mortality and the expected per capita growth rate is greater than one; (2) sink habitat, in which limited reproduction is possible but will not (on average) compensate for mortality, and the per capita growth is between zero and one; and (3) unusable habitat through which animals disperse, which comprises the matrix of all habitats that are never exploited by the species in question and in which patches of source and sink habitats are embedded (Danielson 1992)


• [population regulation theories] The top-down hypothesis focuses on the impacts of predators and/or parasitoids on animal numbers, and the bottom-up hypothesis focuses on the consequences of resource quality.


• The relative effects of predators on herbivore populations varied among seasons and among sites in both years. The impacts of predators on the herbivorous insects were not correlated with known gradients of climatic or of resources quality heterogeneity.


• [supporting bottom up] Herbivorous arthropods were more abundant on creosotebushes on nitrogen-fertilized plots than on creosotebushes on irrigated or control plots (Lightfoot and Whitford 1987).


• Insect abundance was higher on high-nutrient shrubs than on low-nutrient shrubs, confirming in part the bottom-up regulation (Lightfoot and Whitford 1989, 1991).


• These studies show that phytophagous insects on creosotebush are regulated by both top-down and bottom-up processes.


• Roles of Social insects 12


• Ants, for example, are among the most abundant arthropods in most of the world's deserts. - high forager abundance and flexible foraging habits


• Seed-harvester ants of the genus Pogonomyrmexare among the most abundant and widely distributed ants in the Chihuahuan Desert. The numbers of foragers in colonies varied among species: 1,000-6,000 in rough harvester ants (P. rugosus), approximately 1,000 in California harvester ants (P. californicus), and 200-600 in desert harvester ants (P. desertorum) (Whitford and Ettershank 1975).


• Rough harvester ants foraged at night during midsummer and colonies ceased foraging when granaries were filled. Desert harvester ants and California harvester ants exhibited only diurnal foraging behavior and did not exhibit larder-hoarding satiation.


• Foraging in harvester ants was primarily affected by forage availability and secondarily by microclimate (Whitford and Ettershank 1975).


• Scavenging and honey dew collection are other important activities performed by ants (Van Zee et al. 1997). Nearly all small arthropod cadavers that reach the ground are scavenged by ants, particularly the piss ant (ForeliusorIridomyrmex spp.) and the bicolored crazy ant (DorymyrmexorConomyrma bicolor).


• Larger ants and ants adapted to high soil surface temperatures tend to remove these materials over larger distances to their nests.


• The abundance of these two kinds of species at the Jornada leads to scales of redistribution that are nearly seven times that found in the shortgrass steppe biome (Bestelmeyer and Wiens 2003).


• Chihuahuan Desert ant species vary widely in their daily activity patterns


• piss ant and honey-pot ants Myrmecocystus[Endiodioctes subgenus], seed-harvesting ants (Pheidolespp.), long-legged ants (Aphaenogaster [Novomessor]cockerelli), crazy ants (Dorymyrmex[Conomyrma] spp.), piss ants, fire ants (Solenopsis xyloni), perpilosa formica ants (Formica perpilosa), honeypot ants, and New Mexico leaf-cutter ants (Trachymyrmex smithii)


• When long-legged ant colonies were provided both grass seeds and tuna fish, those colonies provided with tuna fish extended their foraging time and remained active until soil surface temperatures reached lethal levels (Whitford et al. 1980a). Colonies provided with seeds ceased foraging at midmorning, the same time that colonies provided with no supplemental forage ceased foraging.


• Most of the ant species responded to availability of preferred forage and to quantities of food stored in the nests. Ant species exhibited satiation when luxury amounts of preferred forage was available, and colonies ceased foraging when satiated.


• [termites] they processed between 3% and 50% of the annual input of detritus and herbivore dung (Whitford et al. 1982) (table 12-1)


• In the Chihuahuan Desert, subterranean termites also consumed large quantities of dead roots of grasses and annuals (Whitford et al. 1988a). There is indirect evidence that termites consume a large fraction of dead roots of shrub species, if termites locate the roots (Mun and Whitford 1998).


• recent studies on termite galleries and sheeting in these habitats suggest that termites may be consuming a larger fraction of the detritus than has been reported for the creosotebush-dominated bajada.


• gut symbionts of some species of termites have the capacity to decompose lignins and other recalcitrant organic molecules (Butler and Buckerfield 1979). Thus, termites produce only small quantities of feces. Termite feces contain very little recalcitrant carbon to contribute to the soil organic matter pool.


• The soil organic matter content of soil patches on a Chihuahuan Desert watershed was found to be strongly negatively correlated with the abundance/activity of subterranean termites (Nash and Whitford 1995).


• Many species of termites have been shown to fix atmospheric nitrogen via hindgut symbionts (Beneman 1973; Schaefer and Whitford 1981; Bentley 1984),


• All ecosystem processes and properties that are modified by the activities of termites taken together make a strong case for considering subterranean termites keystone organisms in Chihuahuan Desert ecosystems.


• [although termites consume from the surface] the decomposition of roots and litter that is buried occurs through the interactions of a complex of soil micro and mesofauna and the microflora.


• Abundance of microarthropods associated with decomposing roots peaked in the warm-wet season (July-September)


• Breeding activity in Chihuahuan Desert mites coincides with the summer rainfall season (Wallwork et al. 1986)


• seasonal breeding pattern was not affected by irrigation during other seasons of the year.


• protozoans and nematodes that are active only in water films on soil particles encyst or enter anhydrobiosis. In desert soils, protozoans and nematodes are in an inactive state most of the time because soils are at soil water potentials of approximately#6.0 MPa much of the year (Whitford 1989).


• microarthropods are the only active microfaunal component of soil food webs during much of the year.


• ants consumed a significant fraction of the seed production of some species of grasses and annual forbs.


• The removal of terminal stems of creosotebush by rabbits results in compensatory growth with several stems originating from below the severed stem (Whitford 1993). Creosotebushes that are pruned by rabbits on a regular basis develop a dense canopy and a hemispherical morphology.


• Predator Ecology 12


• studies of coyotes (Canis latrans) would have been extremely useful, but such studies were not advisable at the Jornada because coyotes were subjected to control practices until the late 1980s.


• The only large raptor that breeds in the Jornada Basin is the Swainson's hawk (Buteo swainsoni). Average density of nesting pairs during the summers of 1974 and 1975 was one pair per 9.4 km2 (Pilz 1983)


• The abundance of ants in the Chihuahuan Desert supports specialized predators: horned lizards of the genus Phrynosoma.


• Simulated predation on rough harvester ant and desert harvester ant colonies revealed that colonies losing approximately 25% of the estimated forager population ceased foraging for up to five days. It was concluded that horned-lizard densities are regulated by the abundance and productivity of Pogonomyrmexants.


• Possibly the most important predator-prey interactions are those in the detrital food webs in the Chihuahuan Desert.


• Many microarthropod species that were thought to be mycophagous were found to be omnivorous. Omnivorous and predaceous mites that prey on bacteriophagous, fungivorous, and omnivorous nematodes are key elements in detrital food webs (Elliot et al. 1988).


• Studies of rodent and rabbit populations in the Jornada Basin have consistently documented low abundance and species diversity in desert grasslands and higher abundance and diversity in the desertified shrublands (Wood 1969; Whitford 1997).


• Wood (1969) reported the rodent biomass in mesquite coppice dune areas (0.72 kg/ha) was double that of the black grama grassland (0.35 kg/ha)


• In places and times of high rodent abundance, rates of herbivory and graminivory may increasingly constrain grass seed production even as grass cover declines (Dabo 1980; Kerley et al. 1997). Rodent cache pits and soil disturbances, on the other hand, may increase the germination rates of some grass species. Thus it is possible that the activities of animals may produce either positive or negative feedbacks on the ecosystem structure, but it is not yet clear which of these effects is most important.


• Conclusions 12


• (1) patterns of shrub cover and water redistribution are dominant elements structuring the environments of Chihuahuan Desert animals, (2) feedbacks from animals influence nutrient availability and plant demography via several direct and indirect pathways, and (3) the contributions of native animals to desertification remains unclear.


• The relationship of conventional notions of degradation to biodiversity involves several mechanisms and is not always clear cut (Bestelmeyer et al. 2003b).


• diversity of indirect pathways that has been uncovered (table 12-2).


• A given taxon may have more than one important effect on ecosystem properties (e.g., limiting N availability but increasing infiltration).


• We do not have enough information to gauge the relative importance of various animal effects for plant recruitment and mortality, especially against a background of livestock grazing, historical legacies, and soil and climate variability.


• 13 Grazing Livestock Management in an Arid Ecosystem


• For four centuries this region has supported a rangeland livestock industry— initially sheep (Ovis aries), goats (Capra aegagrus hircus), and cattle (Bos taurus and Bos indicus), but primarily beef cattle for the past 130 years.


• Seventeen ships carried 1,200 people and enough cattle, horses, sheep, and pigs to colonize northern Hispaniola during Columbus's second voyage in 1493.


• 1521 that Gregorio Villalobos unloaded livestock in New Spain (Mexico) near Tampico; the actual number of cattle and their origin are disputed. [6-50 calves]


• livestock were soon moved north from the Mexico City area during the early sixteenth century with both missionaries and resource extraction industries as retired military officers and Spanish nobility built a mining- and grazing-based economy throughout the region of present-day northern Mexico. By 1539 livestock had reached the present-day United States-Mexico border


• There were a million cattle in New Spain by 1600 (Bowling 1942)


• 1609, the city of Santa Fe was a northern distribution point for livestock in the Americas


• Livestock were given to colonists by the Spanish government as an enticement for settlement (Bowling 1942).


• Individual herds of 4,000-5,000 [sheep] were common throughout the region (Hastings and Turner 1965)


• flocks of sheep were annually driven from present-day northern New Mexico south through the Rio Grande Valley into Mexico to service livestock markets in Chihuahua and Durango (Scurlock 1998).


• The first reports of localized overgrazing by livestock appeared in the 1630s (Ford 1987).


• When Mexico gained independence from Spain in 1821, many of the Spanish settlements were abandoned, and livestock numbers declined. For example, there were fewer than 5,000 cattle in the Arizona territory during the mid-1800s.


• A grazing-based economy was reestablished following the 1848 Treaty of Guadalupe-Hidalgo and the conclusion of the American Civil War in 1865. By 1891, there were 1.5 million cattle in the Arizona and New Mexico territories


• This expansion in numbers was accompanied by an expansion onto rangelands not previously grazed by livestock (Hastings and Turner 1965). This regional exploitation was driven by speculation by Eastern and European investors capitalizing on new technologies for pumping water for livestock and fencing lands (McNaughton 1993). Aggressive programs to control predators and concurrent establishment of railroad networks that moved cattle to growing Eastern markets undoubtedly aided this expansion.


• Cattle numbers in the Southwest peaked at over 1 million head in 1890, during World War I, and again in 1920 but by 1990 had declined to 900,000 head in Arizona and New Mexico (Fredrickson et al. 1998). Currently, forage demand in New Mexico and Arizona is approximately 10 million annual unit months, of which 37% are supplied from federally managed rangelands in these two states (Torell et al. 1992). The regional economy includes a grazing-based component that is predominately comprised of cattle.


• In New Mexico, 9,000+ ranching operations, totaling 600,000 head of beef cattle, generated approximately $800 million in cash receipts from livestock sales in 2001 (USDA 2001).


• The industry is an unconsolidated amalgamation of small businesses with highly variable economic viabilities (Fowler and Torell 1985). Most ranching enterprises have fewer than 250 cattle, employ fewer than 5 people, have been in operation for an average of 19 years, and annually spend $18,000 for community services and $19,000 on structural land improvements (Fowler 1993).


• There are numerous general theories on the role of herbivores in shaping grassland and shrubland ecosystems. These theories include the autogenic hypotheses (NoyMeir 1979/80), optimization theory (McNaughton 1979), evolutionary gradients of grazing history (Milchunas et al. 1988), plant traits adapted to large mammalian grazers (Mack and Thompson 1982), keystone guilds (Brown and Heske 1990), and plant chemical-mediated defoliation (Bryant et al. 1991). None of these theories easily accommodate the inclusion of an exotic large herbivore within an arid ecosystem such as the northern Chihuahuan Desert.


• most of the energy within this ecosystem is traditionally channeled through decomposers rather than herbivores. The black-tailed prairie dog (Cynomys ludovicianus) was an endemic species often found on heavy textured playa soils common throughout the Jornada Basin (Oakes 2000). This species, possibly a keystone herbivore on these playa sites (Miller et al. 2000, and accompanying citations), was poisoned and eradicated prior to and during World War I to reduce forage competition with cattle.


• Prior to extermination efforts, presence of this animal [prairie dog] may have prevented woody plant dominance within more productive desert grassland sites receiving external surface and subsurface water flows (Weltzin et al. 1997).


• Kangaroo rat presence or absence can be more influential on plant community dynamics than the presence or absence of livestock (Brown and Heske 1990). Kangaroo rats also were the targets of private and federal poisoning campaigns during the 1920s to improve forage conditions for livestock. These campaigns were quickly abandoned when the extent of the task was fully realized (Jornada Experimental Range Annual Reports 1925-26 unpublished) and likely resulted in large alterations in the demographics of native mammalian herbivores and their predators for short periods.


• Mature cattle consume 5-15 kg (dry matter basis; NRC 1996) of forage daily. A classic recommendation for stocking desert grassland is 1 cow/260 ha/25 mm precipitation/yr. This stocking level would result in a harvest rate of 7-21 g/m2/yr from an area receiving 245 mm of precipitation. Reported values for forage consumption by cattle under conservative stocking of desert grasslands have been 8-14 g/m2 /yr (Pieper et al. 1983).


• mature cows (average body weight of 328 kg) consumed 6.9 kg (dry matter basis) per day of perennial grasses, of which 82% was black grama. This is an intake rate of 2.1% of body weight per day and a rate that nearly meets the nutritional requirements of a range beef cow in the last trimester of gestation.


• body weight (500 kg) of today's animal unit (AU) and a vastly increased milk production potential, resulting in a 52% greater daily forage intake.


• Jornada desert grassland ANPP during a 3-year period of near average total annual precipitation and protected from cattle grazing ranged from 125-186 g/m2 (Sims and Singh 1978).


• Production with conservative stocking was estimated at 58 (+-20) g/m2 over a 15-year period, which included years of severe drought (Paulsen and Ares 1962).


• we are dealing with a situation in which the primary large ungulate is an exotic domesticated ruminant whose density is directly regulated by humans.


• key aspect of grazing behavior is the expressed forage preferences of livestock. The primary effects of livestock grazing in the Chihuahuan Desert are a function of diet selection.


• Pieper (1994) correctly stated that it could be extremely difficult to predict how livestock will affect rangeland resources because their effects will be highly dependent on the diversities and activities of the grazing animals. Different species and kinds of livestock have different forage preferences


• grazing is species specific (Hobbs and Huenneke 1992)


• annual species are not typically found as major components in cattle diets. Kelt and Valone (1995) reported that only 2 of 79 annual species responded (increased) significantly following livestock removal.


• As with other deserts, understanding individual species responses to defoliation can serve as a good approximation to the understanding of many ecological phenomena in deserts (Noy-Meir 1979/80).


• The primary initial research objectives of the Jornada Range Reserve in 1915 were to quantify the carrying capacity of desert rangelands, establish a system of forage utilization consistent with plant growth requirements, and develop a range management plan to minimize stock loss during droughts (Havstad and Schlesinger 1996).


• A key problem for range management was the inaccurate judgment of carrying capacity (Wooton 1915). Jardine and Forsling (1922), Canfield (1939), and Paulsen and Ares (1962) established guidelines for carrying capacities of black grama rangelands. These classic studies used three different experimental designs to evaluate perennial grass responses to different livestock grazing strategies.


• Jardine and Forsling (1922) evaluated large-scale pasture responses on the Jornada Reserve and adjacent rangeland from 1915-19, a drought period. They measured basal cover responses of black grama to three coarsely applied management practices: (1) heavily grazed yearlong until 1918 and lightly grazed during the 1918 and 1919 growing seasons, (2) grazed yearlong 1915-19, and (3) reduced grazing during the growing season but fully utilized during the dormant seasons, 1915-19. Basal cover responses of black grama, compared to an area protected from livestock grazing, clearly favored treatment 3, and the authors concluded that light grazing during the growing season was the appropriate grazing strategy for black grama dominated rangelands.


• Canfield (1939) conducted a small plot evaluation of black grama responses to different intensities and frequencies of clipping over an 11-year study. In evaluating black grama responses to clipping to either a 2.5 cm or 5 cm residue height at 2-, 4-, or 6-week intervals or once at the end of the growing season, by the end of the study all 1 m2 plots clipped during the growing season were denuded. The obvious conclusion was that moderate or heavy use of black grama over an extended period was inappropriate.


• Paulsen and Ares (1962) summarized observations from small 1 m2 plots arrayed across the JER where basal area of perennial grasses was recorded annually from 1916 to 1953. Plots were stratified to reflect nonuse and light, moderate, and heavy utilization by livestock. Results from this extensive long-term study clearly reflected the need to conservatively (#40% of current year's growth removed) graze black grama and severely reduce or eliminate use during extensive drought periods.


• These authors all concluded that proper utilization of black grama should be less than 40% of current year's growth.


• Jardine and Forsling (1922) recommended the following drought strategies: (1) limit breeding stock to carrying capacities during drought, (2) add surplus stock during good forage years depending on market conditions, (3) adjust range use seasonally depending on growth characteristics of key species, (4) establish permanent watering points no more than 5 miles apart, and (5) establish both herding and salting practices that achieve optimal stock distribution. Similar recommendations for drought conditions are outlined in one of the most current textbooks on range management (Holechek et al. 1998a).


• strategy #5 may have accelerated shrub expansion into areas formerly desert grasslands


• Though livestock dispersal of mesquite seed was seen very early as a reason for mesquite encroachment (Campbell 1929), management practices were not employed to limit further dispersal. Enhancing livestock distribution with stock water and salt placements may have actually promoted mesquite seed dispersal.


• Initial research on livestock production also emphasized strategies for drought. Most of the original efforts focused on supplemental feeding programs, especially those that used locally available foodstuffs, such as cottonseed products


• More novel research has emphasized specialized practices for emergency feed conditions and management of poisonous plants.


• Soapweed (Yucca elata) was found to be a palatable emergency feed when fed chopped and fresh (Forsling 1919).


• Other plant species were either deemed not suitable as emergency feeds (i.e., Dasylirion wheeleri and Yucca macrocarpa) or required spine removal (Opuntia spp.)


• burning spines from prickly pear cactus (in 1924 Forsling estimated that one person could prepare cactus feed for 200-400 had of cattle in a day) was employed during the 1994-95 drought in the Southwestern United States, though not in the Jornada Basin.


• For southern New Mexico, drymaria (Drymaria pachyphylla) became a problem in response to overgrazing in the late 1800s and early 1900s (Little 1937). Drymaria is highly toxic and causes death within hours


• The recommended control practice was hoeing, but eradication was not viewed as a viable possibility. These general characteristics relative to management and control recommendations for poisonous plants persist today (James et al. 1993).


• A few examples of good rangeland conditions under intensive grazing management in the arid zone exist, but these examples are undocumented in the scientific literature.


• The success of these specific situations is probably due to a unique combination of progressive management and a thorough understanding by the ranchers of the ecological characteristics of their specific rangeland.


• general principles of flexible herd management and adjustment of stocking in response to variation in forage production are widely used in many range livestock operations.


• Jornada Basin, Atwood (1987) examined four exclosures in black grama-dominated grasslands after 17, 22, 32, and 48 years of rest. Basal cover of black grama was greater in the 32- and 48-year exclosures compared to adjacent grazed areas. However, no differences between grazed and rested areas were noted after 17 years of rest, and basal cover of black grama was actually greater in the grazed area compared to the exclosure receiving 22 years of rest.


• The use of livestock as biocontrol agents for remediation will require detailed knowledge of this chemically mediated interaction to be an effective technology.


• Conclusions 13


• In summarizing 45 years of grazing research in the arid region of south-central New Mexico, Paulsen and Ares (1961) wrote: "Sustained grazing capacity does not exist on the semi-desert ranges...stocking may be high in some periods (meaning that primary production is high and high livestock numbers would be appropriate) and in others there is virtually no capacity."


• We have a general understanding of the importance of controlling timing, intensity, and frequency of grazing (Holechek et al. 1998b).


• authors and others have repeatedly concluded that proper utilization of arid grasslands should be less than 40% of current year's growth (Campbell and Crafts 1938; Paulsen and Ares 1962; Holechek et al. 1994, 1999).


• problems: (1) coping with temporal variations in forage production, (2) manipulating an animal behavioral process (grazing) that is plant species-specific, (3) managing grazing across landscapes with limited (if any) ability to monitor or assess impacts, and (4) controlling dispersal of seeds.


• Forage production on upland desert rangelands can average between 150 and 250 g/m2 (see chapter 11) during years of normal precipitation but may be#100 g/m2 during drought years (Herbel and Gibbens 1996; see table 11-2 in chapter 11)


• Conservative stocking is probably the most important practice to improve conditions and approach sustained livestock use of New Mexico's arid rangelands.


• 14 Remediation Research in the Jornada Basin: Past and Future


• In 1958, it was estimated that one section (3.2 km2) of black grama grassland could support 18 animal units yearlong, while a similar area dominated by mesquite (Prosopis glandulosa) dunes could support just three animal units (Jornada Experimental Range Staff 1958; see also chapter 13)


• the objectives of the first organized rangeland research in the Southwest were to identify proper techniques to restore grasslands that had been overgrazed (Jardine and Hurtt 1917; Havstad 1996).


• Today, we recognize the importance of multiple, interacting factors in addition to overgrazing, and research is more broadly focused on the recovery of ecosystem functions necessary to support multiple ecosystem services.


• The Society for Ecological Restoration considers that "an ecosystem has recovered when it contains sufficient biotic and abiotic resources to continue its development without assistance or subsidy. It will demonstrate resilience to normal ranges of environmental stress and disturbance. It will interact with contiguous ecosystems in terms of biotic and abiotic flows and cultural interactions" (Society for Ecological Restoration Science and Policy Working Group 2002).


• Although restoration of perennial grasslands is often cited as the ultimate objective of management intervention in the Southwest, we recognize that in many if not most cases complete restoration of a preexisting plant and animal community is impossible


• Historic Approaches 14


• The first began with the creation of the Jornada Experimental Range (JER) and emphasized improved livestock management. The second was associated with availability of inexpensive labor during the Great Depression of the 1930s, coinciding with the recognition that livestock management alone might be insufficient to reverse shrub encroachment into grassland. The third period was dominated by increased reliance on herbicides and mechanized shrub control.


• A number of state and federal experiment stations were established at the end of the nineteenth and beginning of the twentieth century. Research was designed to estimate the carrying capacity of rangelands during drought and nondrought years, to evaluate and demonstrate the use of water, mineral feeding stations, and fencing to control livestock distribution and increase the quantity and quality of meat production. A number of livestock exclosures were established, and in some cases, clipping trials were initiated to determine sustainable levels of plant utilization.


• On the JER, four large (250 ha each) exclosures were established during the 1930s: the natural revegetation, the mesquite sand hills artificial revegetation, the gravelly ridges (creosotebush [Larrea tridentata]) artificial revegetation exclosure, and the Dona Ana moisture conservation plots exclosure.


• general trend was increased shrub dominance


• mesquite-black grama ecotone. It is now completely covered by mesquite duneland


• [modern day] GPS and GIS technologies. These technologies will allow site-specific management of relatively small areas in extensive rangeland systems without fencing (Anderson 2001; see also chapter 13)


• failure of livestock management alone to reverse shrub invasion was beginning to be recognized by the 1950s (JER Staff 1958).


• now clear that some degraded sites will not be remediated simply by exclusion of domestic livestock (Bestelmeyer et al. 2003a; see also chapter 10).


• Civilian Conservation Corps (CCC) provided a large supply of labor during the Depression years of the 1930s. There were approximately 50,000 CCC workers in New Mexico between 1933 and 1942 (Melzer 2000).


• Few detailed records of the manipulations completed by the CCC have been preserved, but many of the structures and patterns created can be detected both on the ground and in aerial photographs dating to 1935 (figure 14-1; Rango et al. 2002), and the objectives and results of some of the experiments are summarized in internal reports.


• Many of these individuals continued with careers in rangeland management after World War II (Ares 1974) [P.A. Yeomans began his land work after WW2]


• [1930s] Various approaches were tried to enhance grass establishment. The treatments can be classified into five basic groups: seeding and transplanting, microsite manipulations, shrub removal, water redistribution, and small mammal control. Livestock were excluded from most of the experimental areas.


• Although many of the manipulations appeared to increase grass establishment temporarily, few had significant lasting effects on plant community composition. Some of the treatments applied in the 1930s could no longer be detected in aerial photographs by 1968, whereas others were still visible in 1996 (table 14-2).


• Seeding and transplanting were generally done in association with one of the other treatments.


• Fourwing saltbush (Atriplex canescens) was one of the most popular species on sandy basin soils because of its value as a forage crop and its observed potential to compete with or at least coexist with mesquite.


• Valentine (1942) concluded that success depended on rainfall distribution and amount during the establishment period and during subsequent years. Many of the seedings in the artificial revegetation exclosure were planted in 1934, a year with barely half the long-term average rainfall.


• [Microsite Manipulations were]designed to improve conditions for seedling establishment at the microsite scale (Fowler 1986) included piling brush (often cut from the top of mesquite dunes), digging trenches (to trap soil and seeds), and cutting roots (to reduce root competition from mesquite).


• Seed burial was cited as a problem in many cases, particularly in the interdune areas.


• Grubbing (shrub removal using hand tools) was frequently cited as one of the most effective approaches for maintaining or increasing the productivity of perennial grasslands. Attempts to combat shrub invasion by hand continued well into the second part of the twentieth century. As late as 1958, Herbel et al. concluded "grubbing light stands of young mesquite plants is the most economical means of controlling mesquite" (Herbel et al. 1958).


• Grubbing becomes more difficult as plant size increases because the root crown must be removed to prevent resprouting. Tarbush (Flourensia cernua) and creosotebush, relatively weak sprouters compared to mesquite, were sometimes simply cut off at the soil surface.


• Water redistribution was attempted at a variety of scales using both hand tools and tractor-mounted implements (table 14-1). In most cases, the objective was to slow the movement of water across the landscape through the creation of soil dikes, terraces, or furrows (figure 14-2). In at least one case, brush dams were constructed between mesquite dunes in areas with a gentle slope. Linear brush piles were also used to spread water from rock dams in gullies across creosotebush-dominated slopes.


• Limited data available from a few plots showed only partial, if any, grass response to these structures by the mid-1940s (Valentine 1947). In a review of the water redistribution projects of the 1930s, Valentine (1947) reported that "In general it is impossible to identify any area either above or below the spreaders that have been benefited from them. This is true even of the area above the spreaders where the marks left by standing and running water give evidence that they were instrumental, at least occasionally, in bringing water to and holding it on these limited areas." Interestingly, however, many of these treatment areas are visible six decades or more after their establishment due to higher grass or shrub cover and/or changes in species composition.


• These long-term changes are often associated with a persistent shrub response to many of the water redistribution treatments.


• The concentration of even relatively limited quantities of water and nutrients on the contour terraces and behind the dikes could explain the apparently higher shrub biomass visible in figures 14-1 and 14-2. Valentine (1947) also observed that the brush water spreaders created more favorable microsites for seedling establishment. This observation is supported by Gutierrez-Luna (2000) who showed that water-dispersed seeds are preferentially deposited in naturally formed litter dams, and moisture and temperature conditions in these microsites tend to be more favorable for seedling establishment. Seedlings survive longer where there is litter on the surface.


• Most of the water-retaining structures constructed during the 1930s were built on relatively coarse-textured soils with slopes#2%. In at least one case in which soil moisture content was measured, the water-holding capacity of the soil was so low that no increase in moisture availability was detected behind the ridges (Valentine 1947). Additional water-retention structures were built in the 1970s on heavier soils with lesser slopes. Unlike the water-retention structures built in the 1930s, which were in shrub-dominated areas, the 1970s contour dikes were established in areas that were devoid of vegetation. The 1970s structures were abandoned within four years due to high maintenance costs and limited grass establishment in spite of seeding and sewage sludge applications. Twenty-two years later, however, native species had revived (Walton et al. 2001; see also figure 14-2).


• These data, together with the fact that none of the water-retention structures constructed during the 1930s was maintained for more than five years, suggest that these strategies might be better viewed as being only partially tested rather than rejected as failures. In fact, some of the most vigorous grass patches on the Jornada are located upslope of the most carefully maintained water redistribution structure in the Basin: the access road that connects all of the CCC camps and the JER headquarters to the city of Las Cruces. These patches extend up to 20 m upslope from the road, and there are a number of areas in which production is correspondingly reduced downslope.


• [Small mammal control] poisoned grain (for rodents) and poisoned salt blocks (for rabbits)


• Valentine (1947) suggested that rodent and rabbit damage was one reason for the failure of brush dams to increase grass establishment between mesquite dunes


• The successful efforts to remove prairie dogs (Cynomys ludovicianus) from the basin in 1916-17 and the unsuccessful attempts to eradicate kangaroo rats were part of early remediation attempts and were apparently based on the assumption that these animals competed extensively with livestock for forage


• While small mammals reduce perennial grass density near their burrows and increase the wind- and water-erodible soil and in unvegetated patches (chapter 12), they also remove shrub seedlings and are important seed dispersal agents that may actually contribute to future remediation strategies (Havstad et al. 1999).


• For example, Gibbens et al. (1992) reported that 81% of mesquite seedlings emerging in response to a July 31, 1989, rainstorm were dead by the following May. Of these, all but 2% had been bitten off "at or slightly below the cotyledonary node, which causes death." All of the surviving seedlings also showed signs of herbivory.


• The Promise of Technology (1941-1980s) 14


• During the boom years following the war, fossil fuels replaced human labor as the most cost-effective input and "shrub control" became the new mantra. The authors of the 1958 annual report for the JER observed that "there is no evidence of recovery [of mesquite sand dunes to grassland] after 25 years, even in areas completely protected. To the contrary, severe duning has been spreading even with conservative grazing...the absence of grazing use will not retard that spread . . . The suggested control method is grubbing" (JER Staff 1958).


• The reduced emphasis on water redistribution-based approaches attempted during the 1930s was probably driven in part by the absence of any evidence of positive impacts of these approaches during the drought of the 1950s, when annual precipitation (from 1950-56) averaged just 150 mm/year.


• Although there are few quantitative records of the effects of the Depression-era treatments, the quadrat studies (Gibbens and Beck 1987) and shrub removal/lagomorph exclusion experiments suggest that grass recovery in response to management was virtually erased during the accelerated loss of grassland during the 1950s drought.


• Herbicide studies were initiated in the 1930s. Plants were sprayed with sulfuric acid, kerosene, sodium chlorate, and diesel oil and dusted with mixtures of borate and sodium chlorate.


• The development of phenoxy herbicides, specifically 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), after World War II opened a new era in shrub control.


• could be applied aerially over large areas and, with timely application, could result in significant reductions in shrub productivity and density. Although 2,4,5-T can no longer be legally applied to rangeland, there are a number of other materials, including clopyralid, tebuthiuron, and monuron, that are still available.


• Shrub density has been successfully reduced on relatively large areas in the Chihuahuan Desert in both the United States and Mexico, and herbicide applications are frequently included in brush management plans. Much of the remaining grassland on the JER and the Chihuahuan Desert Rangeland Research Center (CDRRC) has been treated at least once with herbicide.


• New Mexico State University Agricultural Experiment Station Bulletin (Herbel and Gould 1995). Even this relatively optimistic publication, however, concludes with the cautionary note that "it is possible to renovate brush-infested rangelands with herbicides, but some of the practices are costly."


• [mechanical] treatments result in high levels of soil surface disturbance, increas-ing erosion susceptibility (Wood et al. 1991). They also require significant energy inputs and the availability and maintenance of expensive machinery.


• Holechek and Hess (1994) estimated that burning cost $2.50-$12.50/ha (where sufficient fuel exists), herbicide cost $30-50/ha, and mechanical control cost $60-125/ha. Estimates include the cost of materials, machinery depreciation, and labor.


• Jones and Johnson (1998) pointed out that some of the failures may have been unnecessary as, "sophisticated analyses of ecological adaptation and genetic variation were rarely considered in early trials." In other words, at least some of the failures may be partially attributed to the seeding of species and varieties that were not adapted to the local edaphic and climatic conditions.


• other technologies have been tried, ranging from applying hot wax to increase runoff from mesquite dunes (Gibbens personal communication) to using polyacrylamide to reduce soil crust resistance to seedling emergence and increase infiltration capacity.


• Data Quality and Reliability of Conclusions 14


• many records no longer exist, others persist in archives scattered throughout the country.


• there are enough successes to convince some, at least, that the system can be controlled (Cassady and Glendening 1940)


• it often takes decades to determine the success or failure of a particular manipulation. It may take decades for positive effects to appear as plant-soil feedbacks gradually change soil waterholding and infiltration capacities.


• start of the International Biology Programme (IBP) in 1970 , most research in the Jornada Basin was specifically designed to determine which treatments could be used to improve management. Over the past three decades, emphasis on improving understanding of basic ecosystem processes has increased.


• future management systems will depend on a more thorough understanding of these processes, and it is not always obvious which need to be studied


• 1986, Wright and Honea "It seems as if the entire set of changes in the soil environment...ensure that mesquite will occupy those sites for long periods of time."


• Thirteen years after velvet mesquite (Prosopis juliflora) removal, canopy-interspace differences in soil carbon were virtually identical to those where mesquite had been left intact at a site in southeastern Arizona, which has slightly higher temperatures and winter rainfall than the Jornada Basin (Tiedemann and Klemmedson 1986).


• Schlesinger et al. (1990) also highlighted the importance of the formation, maintenance, and deterioration of resource islands in deserts throughout the world. Studies in Israel (Boeken and Shachak 1994), and Australia (Tongway and Lud-wig 1997), as well as the United States (Valentine 1941; Schlesinger et al. 1996; Wainright et al. 1999b), have addressed the mechanisms by which human- and vegetation-formed patches affect plant production and resource availability and redistribution at multiple spatial scales.


• This trend toward understanding the processes responsible for the patterns should help target those processes that limit the success of remediation attempts.


• Understanding the importance of resource redistribution at the plant-interspace level generated by the Jornada Basin research has led to a renewed interest in the possibility of manipulating resource availability to trigger changes in vegetation composition and structure. Most now agree that one of the keys to the persistence of shrublands in spite of diverse efforts to remove them is their ability to acquire resources from both greater depths and larger areas than grasses and to concentrate and retain those resources in self-reinforcing islands of fertility (figure 14-3; Valentine 1941; Wright and Honea 1986; Schlesinger et al. 1990). The key to maintaining production during drought years is the ability to tap deep water, while extensive shallow-root systems allow shrubs to compete with grass for water from brief or low-intensity rainstorms (Gibbens and Lenz 2001). The effect of reduced nutrient availability on grass production in mesquite dune interspaces was documented over 60 years ago (Valentine 1941).


• McAuliffe (1994) documented strong relationships between soil development and vegetation in southeast Arizona. Similar relationships have been reported for the Jornada


• Perennial grasslands tend to persist on soils with an argillic horizon near but not at the soil surface. Argillic horizons are rich in clay and tend to retain more water at a depth that is accessible to grass roots.


• Similar patterns have been documented in the Chihuahuan Desert. Where black grama grasslands persist on coarse-textured soils in the Jornada Basin, there is often a calcic or petrocalcic horizon near the soil surface (Teaschner 2001). Highly developed calcic horizons appear to be relatively impervious to both water and roots. Most of these horizons, however, are heavily invaded by roots and have higher water-holding capacity than the loamy sands typical of many Chihuahuan Desert basin soils now dominated by mesquite.


• The failure of at least one intensive effort to increase grass establishment by concentrating water was attributed to the fact that the water-holding capacity of the soil was too low to support grassland (Valentine 1947).


• The importance of lag time between climate and plant community changes associated with soil-vegetation feedbacks is poorly understood.


• some plant communities, such as those in playas, rely on larger, rarer events that result in water redistribution at the landscape scale. We do not understand how changes in the intensity and frequency of these redistribution events affect plant establishment, production, and survival at the two scales. We know even less about the effects of nutrient redistribution at multiple scales.


• Seed banks represent yet another form of dispersal, but in time instead of space.


• attempt to reestablish grassland in currently shrub-dominated systems [why is the focus only on going back to grasslands? Could making it a forest etc be more productive?]


• W. G. Whitford (personal communication) estimated that honey mesquite produced over 100 seeds per square meter in a mesquite-dominated community based on seed and pod counts.


• Mesquite seeds are dispersed over relatively short distances by rabbits and other small mammals and over longer distances by livestock (Tschirley and Martin 1960).


• some evidence to suggest that fire may be used in the Chihuahuan Desert to limit shrub expansion if it is applied during a relatively wet year when grasses can recover. The effects of a burn in most systems depend on careful timing relative to current weather, soil moisture, fuel load, and the size and growth stage of both the herbaceous and woody components (Drewa and Havstad 2001).


• one of the most important factors is also among the least predictable: the weather conditions following the fire.


• [2 studies] fire reduces mesquite shrub volume but does not result in shrub mortality.


• Shrub volumes declined 40% in the burned areas and increased 30% in the unburned controls, but there was little evidence of shrub mortality (Drewa et al. 2001).


• although fire may help slow shrub invasion, benefits, [high] costs, and potential risks should be carefully considered before application to large areas.


• Future Scenarios 14,


• Restoration of native grasslands and other plant communities may be limited by one or more of the following factors: <OL> invasion of highly competitive, persistent species; loss (both documented and undocumented) of plant, animal, and microbial species from the system; loss of soil and/or modification of dynamic soil properties; infrequency of suitable establishment periods with adequate soil moisture; landscape level processes that overwhelm small-scale manipulations; and those sites that are the most resilient will not necessarily be the most resistant to future degradation. </OL>


• To increase the probability of success, we need to focus more on restoring the resistance and resilience of soils and plant and animal communities rather than on short-term similarities to a particular community structure or composition.


• Future success will also depend on recognizing that neither the soils nor the climate nor the faunal community are the same today as they were during the latter part of the nineteenth century when shrub invasion into grasslands began to accelerate. They are likely even more different than they were when the perennial grasslands became established. Loss of soil and animal species and additions of new species to the system, together with climate change, may mean it is no longer possible to reestablish some plant communities or that it may be possible to reestablish them only in selected parts of the landscape.


• authors of a 1939 report argued that although many of the intensive revegetation approaches "would be uneconomical for general application...certain key or strip areas could be so treated on the basis that the entire area would in time be improved by natural spread from the treated portions" (JER Staff 1939).


• The concept of targeting areas with naturally higher resource availability was resurrected by Herbel (1982) in a proposal to focus herbicide-based brush control on run-in areas, while leaving shrubdominated upland areas to produce runoff. The argument was based both on the low benefit-cost ratio for treating the upland, runoff-producing areas, the higher returns for the run-in areas, and the recognition that maintaining runoff from the upland areas could be used to increase production on the lower areas.


• trigger sites [key areas to focus/start on]


• Degraded riparian zones in the Southwestern United States, for example, recover relatively quickly compared to upland systems when livestock, and sometimes elk (Cervus), are excluded except when invaded by salt cedar (Tamarix). However, remediated systems are often not very resistant to subsequent overgrazing until the woody species have grown beyond the reach of livestock, though much of the research in this area is anecdotal (Sarr 2002).


• All of these limitations can potentially be overcome through a combination of: (1) careful analysis to identify the factors and processes that are most likely to limit establishment and survival at a particular time and location; (2) patience and a flexibility to initiate interventions when they are most likely to be successful, rather than when funding and logistical support are available; and (3) attention to landscape-level controls.


• Creative integration of multiple approaches, including short- and long-term experiments and monitoring, gradient analyses and descriptive studies, and conceptual, empirical, and theoretical modeling, will be necessary to develop effective remediation strategies based on an understanding of key ecological factors and processes (Archer and Bowman 2002).


• Relevance to the Southern Chihuahuan Desert and Other Deserts 14


• The basic patterns and processes described here are similar to those described for many other parts of the world. Roundy and Biedenbender (1995) draw similar conclusions based on their review of literature primarily focused on Arizona. Balli´n Corte´s (1987) identified a similar suite of limitations to the recovery of ecosystems in his analysis of desertification in the southern Chihuahuan Desert at approximately 21# N latitude. Lovich and Bainbridge (1999) concluded their assessment of the potential for restoration of southern California deserts by stating that "restorative intervention can be used to enhance the success and rate of recovery, but the costs are high and the probability for long-term success is low to moderate." The recommended strategy for the future is similar to that proposed by Whisenant (1996) and Tongway and Ludwig (1997), based on their experiences in central Texas and Australia, respectively.


• Conclusions 14


• objective of increasing the resistance and resilience of the modified ecosystems.


• remediation efforts through the 1980s were primarily designed to produce more forage for livestock (a societal benefit) and to reduce soil erosion (a cost). The success of remediation efforts, however, has been generally defined usually in terms of the net economic benefit to livestock producers. Evolution of societal values, human population growth, and the associated redistribution of financial resources will inevitably lead to shifts in the ways that costs and benefits of remediation efforts are evaluated.


• Social and political values help define how ecologists view ecosystems, what we decide to emphasize in our research, and how we describe the results of our research. [how they describe results?!]


• ongoing debate on the relative importance of changes in root versus fungal biomass (Hodge 2001). Both are difficult to measure accurately, and biases can easily result in the selection of measurements, techniques, or levels of replication that favor one variable over another.


• 15 Applications of Remotely Sensed Data from the Jornada Basin


• in 1995 we began to collect remotely sensed data from ground, airborne, and satellite platforms


• In general, as ground cover decreased from grass to transition to mesquite communities, reflectance measured from all instruments increased


• data suggest that changes of vegetation from grass-dominated to mesquite-dominated communities in the Jornada Basin could have significant effects on the albedo and the surface temperatures measured at the different sites and on the overall water and energy budget within the basin.


• Leaf area index (LAI)


• mesquite site LAI has no consistent pattern, whereas the creosote site generally has a greater LAI in spring. LAI was low at all sites in September 2001. [probably terrorists] Variability in the range of LAI has increased since 1995 with great fluctuations in measurements in recent years.


• Mesquite communities have the highest reflectance with the exposed soil cover fraction up to 0.75. The transition communities have the next highest radiance, followed by the grass communities. The creosote and tarbush communities are usually between the grass and transition communities.


• Wind speed and direction were measured using a Met One anemometer and wind vane located at a nominal height of 3 m above the local topography


• Soil moisture was measured at over 5 cm depth using three soil moisture resistance sensors


• By the early 1930s, the USDA started to systematically photograph agricultural lands in all states. Black-and-white aerial photographic coverage over parts of the Jornada Basin began in 1935. [and continue today]


• Landsat was the pioneering Earth resources technology satellite


• Landsat has been in existence since 1972 [it is the standard]


• [other satellites] Ikonos and QuickBird


• 0.61-m, panchromatic (or 2.44-m multispectral) resolution of the commercial QuickBird satellite


• alternatives like the use of Unmanned Aerial Vehicles (UAVs) are currently being explored


• Eventually, resource management agencies, rangeland consultants, and private land managers should be able to use small and lightweight UAVs to acquire improved data at a reasonable cost


• To get a landscape perspective at reasonable cost without mosaicing, Landsat TM data seem to be ideal. The entire Jornada Basin is covered in one frame.


• When very high-resolution imagery is not the primary goal, Landsat TM data are a very useful product for rangelands. The current Landsat platforms are old and becoming unreliable, so it is critical that follow-on Landsats be designed and launched very soon.


• 16 Modeling the Unique Attributes of Arid Ecosystems: Lessons from the Jornada Basin


• Jornada Basin is typical of arid ecosystems of the Southwestern United States and many other regions of the globe


• arid ecosystems are structurally and functionally quite complex


• striking spatial and temporal heterogeneity in the availability of essential limiting resources, such as water and mineral nutrients (MacMahon and Wagner 1985; see also chapters 5 and 6)


• majority of our efforts have been devoted to analyses of ecosystem carbon, nutrient, and water dynamics using the patch arid lands simulator-functional types model (PALS-FT) (table 16-1)


• PALS-FT consists of four principal modules: (1) soil water distribution and extraction via evaporation and transpiration; (2) soil, surface, and canopy energy budgets; (3) plant growth, including phenological and physiological responses of key principal plant functional types; and, (4) nutrient cycling, including soil organic matter, decomposition, availability of inorganic N.


• productivity is a manifestation of interactions among numerous soil, plant, and atmospheric variables that result in complex patterns of soil water storage and water use by plants (chapter 11)


• Our proposed revision of the pulse-reserve model (compare figure 16-2a and 16-2b) is not meant to be all-encompassing. Rather, it serves as a general guide for identifying those key processes


• late 1800s the Jornada Basin consisted largely of warm-season, C4, perennial grasses; a century later, these communities have largely been replaced C3 shrub-dominated communities


• A series of connected patches form flowpaths (schematics provided in Reynolds and Wu 1999). Distinct geomorphic surfaces (alluvial fans, piedmonts, etc.) or topographic features (e.g., watersheds) often form natural boundaries for flowpaths (chapters 2, 4, and 7)


• variable seasonal rainfall, downslope redistribution of water and organic matter, and soil texture-related variation in infiltration and water-holding capacity all interact to generate a complex spatial and temporal gradient of patch types with differing soil water and nitrogen availabilities.


• plants and most other ecosystem components (e.g., decomposers) respond primarily to soil water, not precipitation per se (see chapter 5).


• depth-dependent distributions of soil water (often due to relatively impermeable layers of calcium carbonate or clay)


• whole plant dynamics are modeled via descriptions of organs (e.g., allocation between roots and shoots, leaf photosynthesis), whereas community dynamics is modeled based on whole plant dynamics (for examples, see Reynolds and Leadley 1992).


• A single precipitation event may affect plant survival and growth, but the effect is dependent on its timing relative to other events (Reynolds et al. 2000).


• scales of observation (patterns), theory (explanations), and models (mechanistic descriptions involving key processes, interactions, and feedbacks) (Reynolds et al. 1993). Our goal is to develop models capable of prediction at the level of interest while avoiding scaling errors.


• Although the small fraction of active roots in the surface layers normally contribute little to total plant water uptake, their role may be to acquire water immediately following a rain event (BassiriRad et al. 1999). For example, nearly 30-60% of the water taken up during the first and second days following a 17-mm rainfall came from the top 10 cm (figure 16-7b, c). This water is probably critical to creosotebush growth and carbon dynamics because photosynthesis is often enhanced immediately following rain events (Reynolds et al. 1999b). Essential nutrients are also concentrated in the topsoil (Jobba´gy and Jackson 2001), and the small fraction of active roots near the surface may allow creosotebush to capitalize on improved nutrient availability following rain (BassiriRad et al. 1999).


• Patch Scale: Soil Water and Nutrient Cycling 16


• All three models predicted lowest transpiration (about 40% of total ET) for the creosotebush community (Station 65, figure 16-3) with the lowest plant cover (30% peak cover) and highest transpiration (58-70% of ET) for the mixed vegetation. However, these differences in transpiration were also a function of soil texture differences


• Experimental studies of ET have shown that the percentage of total ET attributable to transpiration varies from 7% to 80% in various arid and semiarid ecosystems in North America (reviewed in Reynolds et al. 2000).


• An important "take-home lesson" is the importance of the interdependency between transpiration and evaporation as a result of competition for soil water between the atmosphere and the plants. Our modeling research suggests that studies attempting to study transpiration or evaporation in isolation from one another are likely to reach erroneous conclusions


• our models suggest that plant cover and rooting distributions are more important in determining soil water distribution than physical processes.


• Litter and/or nutrient inputs are sporadic (Crawford and Gosz 1982), and both litter and nutrients are usually spatially heterogeneous, reflecting heterogeneity in the vegetation (Schlesinger et al. 1996)


• results also suggested that decomposition of litter was strongly limited by N availability to microbes during the early phases of decay


• Although we intuitively expect that NPP in arid lands should be directly related to rainfall, we find that interannual variation in NPP for a given arid land site is, in fact, only weakly correlated with precipitation (Paruelo et al. 1999; Oesterheld et al. 2001; Wiegand et al. 2004). Le Houerou et al. (1988)


• indicating a complex relationship that defies simple conceptual models


• the response of well-hydrated plants to a given moisture input will be different than drought-stressed plants (e.g., BassiriRad et al. 1999), and some arid land species require a minimal precipitation event to trigger a transition from a state of lower to higher physiological activity (Schwinning and Sala 2004).


• Given the ecological significance of how seasonal pulses of moisture could be vertically separated into shallow and deep soil water pools - which can be differentially utilized by shallow-rooted grasses and deep-rooted woody plants (i.e., the twolayer hypothesis)


• the majority of all individual precipitation events in the Jornada Basin are less than 5 mm, we hypothesized that small events may be "amplified" to some extent if they were to occur on sequential days


• In general, our simulations revealed many nonlinear responses. Large events and storms generally elicited much greater growth responses than small ones


• The aboveground biomass of severely drought-stressed grasses tended to decline following any size rain event, reflecting the reallocation of biomass from crowns and shoots to new root growth (see Reynolds et al. 2004


• there was a difference in depth of soil water recharge depending on soil texture. For medium to fine-textured soils ( greater than 18% clay) there was no consistent recharge below 60 cm, whereas there was consistent (2/3 of years) recharge below 60 cm for coarse-textured soils ( less than 12% clay).


• results indicated that there was little vertical water partitioning among the plant functional types (figure 16-10). For both soil types, the largest water use was from the top 20 cm, which provided 35% of the water transpired for the coarse soil and 46% of the water transpired for the fine soil.


• we could hypothesize that if overgrazing caused a shift from grass- to shrubdominated plant communities, the ratio of transpiration to ET (T/ET) would be reduced because of greater evaporation associated with increased bare soil.


• the simulation showed that over a 100-year period, the average T/ET was 34% for boththe grass- and shrub-dominated communities (using Las Cruces, NM, weather data; see Reynolds et al. 2000). On the other hand, there were large annual differences in total and seasonal transpiration for a particular patch type and for a given year.


• a year with a large amount of precipitation in midsummer would favor growth (transpiration) of C4 grasses and summer annuals, whereas a year with a large amount of precipitation in winter and early spring would favor evergreen shrubs and winter annuals.


• Hunter's (1991) analogy for warm desert systems that most of the plants are "drinking from same cup with different straws," but as our simulations show, not always at the same time.


• the Jornada Basin and other desert regions may be characterized by roughly decadal-length periods of drought or above average rainfall (Conley et al. 1992; Reynolds et al. 1999a)


• total annual NPP is considerably greater than the variability in rainfall both within and between decades (table 16-7).


• The 33% decline in rainfall during the dry decade resulted in a nearly 40% decline in NPP, whereas the 32% increase in rainfall during the wet decade caused a 300% increase in NPP.


• the decade of summer drought impacted summer-active C4 perennial grasses much more than C3 shrubs, or other winter/spring-active plant functional types. But these simulations also produced a somewhat counterintuitive result: that C4 grasses would also be most responsive to shifts in late winter or spring rainfall. These findings contrast with a study at a nearby site in the Chihuahuan Desert, in which Brown et al. (1997) reported that C3 shrubs had the greatest response to increased winter/spring rainfall over nearly the same period (1979-92) as for our simulation. The shifts in precipitation over this period were apparently different across the northern Chihuahuan Desert (Brown et al. 1997), suggesting that relatively subtle shifts in seasonal precipitation patterns could elicit relatively large differences in ecosystem responses across these arid land systems


• hypothesis that a relatively delicate balance exists between grass- versus shrub-dominated ecosystems, which can be tipped in part by seasonal shifts in precipitation (Reynolds et al. 1997).


• We found the effects of runoff/run-in redistribution on plant responses and soil water dynamics to be generally important and, in several instances, dramatic


• Our simulations generally support the hypothesis that an increase in the number of large precipitation events may favor shrub establishment and growth


• Natural and human disturbances will continue to affect the Jornada Basin and other arid ecosystems of the globe in unknown and complex ways. In spite of much progress, significant gaps remain in our knowledge


• arid ecosystems, which consist of copious slow processes that may abruptly switch their rates of change in response to changing environmental drivers. Responding to these and other concerns, we have previously noted that this leads to a somewhat troubling paradox: in the absence of data and understanding, there tends to be a heavy reliance on models, the quality of which are in turn highly dependent on the quality and availability of data and understanding (Reynolds et al. 1996B, 2001).


• It is important to recognize that although we should not necessarily trust models to accurately predict arid land responses to climatic change or other human perturbations, model simulations can help us understand ecosystem functioning and indicate how sensitive arid lands may be to projected human impacts. As our knowledge improves, our models will improve.


• 17 A Holistic View of an Arid Ecosystem: A Synthesis of Research and Its Applications


• The Jornada Basin LTER was established in 1981 with the primary aim of using ecological science to understand the progressive loss of semiarid grasslands and their replacement with shrublands. This motivation echoed that which initiated the Jornada Experimental Range (JER) 69 years earlier.


• The research questions first asked by the U.S. Forest Service and later by the Agricultural Research Service (ARS), such as how to manage livestock operations, frame much of the Jornada Basin research.


• Within the basin, the patch has served as the fundamental unit of organization. The patch includes plants and their associated interspaces (Schlesinger et al. 1990).


• Disturbances and regeneration of vegetation lead to changes in patch identity (e.g., a grass or shrub patch) and location over time (White and Pickett 1985).


• Past and current Jornada research suggests there are general rules by which patch mosaics (and their effects) are organized within landscapes via geomorphic patterning (Ludwig and Cornelius 1987; McAuliffe 1994; Wondzell et al. 1996; see also chapter 16).


• Plant-Soil-Animal Feedbacks Govern Patch Transitions 17


• There is historical evidence that variability in the magnitude and coincidence of multiple stressors, particularly extended drought periods cooccurring with instances of overgrazing by livestock, have led to episodic losses of grass patches. These periods include years during the early 1890s, 1910s, 1930s, and 1950s (chapters 10 and 13).


• Summer droughts associated with increased frequencies of El Nino periods (featuring high winter rainfall) over the past century may have favored shrub establishment and survival at the expense of perennial grasses (Brown et al. 1997; see also chapter 3)


• It is unclear why particular patches of certain species (e.g., black grama,Bouteloua eriopoda) are lost while others of the same species survived during a given drought episode (Gibbens and Beck 1988).


• may also lead to a grass-shrub symbiosis when grass cover is low because shrubs create stable microenvironments for grass establishment and persistence. Understory grasses further reduce raindrop impact and promote local infiltration (Abrahams et al. 2003).


• Differences in C and N cycling patterns can be viewed as both consequences and drivers of vegetation change.


• interplay of rainfall patterns, soil degradation, and variable nutrient limitation in space and time may regulate the pace of vegetation change.


• potential activity of termites, which are the major animal contributors to nutrient cycling, is little affected by changes associated with grass-shrub transitions (chapter 12)


• Only extreme soil degradation associated with the formation or exposure of cemented soils in shrub interspaces seems likely to restrict termite activity.


• The patch-level consequences of biotic effects on nutrient cycles are as yet only partly understood.


• Changes in vegetation physiognomy associated with shrub encroachment may favor populations of rodents and lagomorphs, leading to increased herbivore pressure on seedlings and the reproductive parts of adult grass and shrub plants (Nelson 1934; see also chapter 12). This effect may limit plant recruitment (figure 17-1). On the other hand, increased small mammal densities may increase rates of biopedturbation, improve rates of water infiltration in interspaces, and increase the likelihood of seed germination (Whitford and Kay 1999). For a given patch of mesquite shrubland, it remains unclear (1) whether biotic or abiotic limitations to grass recovery in shrub interspaces are most important and (2) whether particular taxa, such as small mammals, have a net positive, negative, or neutral effect on grass abundance at the patch scale.


• aboveground net primary productivity (ANPP) data in grasslands and shrublands suggest that reconfiguration of biological activity has not led to reductions in energy capture (assuming similar initial potential) at broader scales. A shift from grass to shrub dominance appears to involve changes in the identity of the producers, rather than a significant change in the overall ANPP production rates (Huenneke et al. 2002; see also chapters 5 and 11


• existing measurements do not consider belowground productivity, it is possible that total productivity (TNPP) and carbon sequestration are greater in shrublands than in grasslands (see House et al. 2003). Existing data, however, suggest that grasslands and shrublands have similar efficiencies with respect to the use of N and water, despite strong differences in how these nutrients are acquired (Reynolds et al. 1997; see also chapter 8


• Three vectors have been examined in detail at the Jornada: wind, water, and animals.


• Vegetation bands or "stripes" may be produced on the gentle slopes and loamy soils of lower piedmont positions (see Aguiar and Sala 1999).


• rainfall events producing significant run-off lead to surface water transfers among landforms (Phillips et al. 1988; see also chapter 7). These transfers may be critical determinants of plant community patterns. Run-in water may be a significant factor in maintaining productive tobosa (Pleuraphis mutica) grasslands in lower piedmont and marginal basin floor positions (Herbel and Gibbens 1989; see also chapter 6)


• Historical decreases in grass cover in upslope positions may have allowed increased surface water runoff, resulting in increasing cover downslope over the same period (Herbel et al. 1972).


• redistribution of nutrients and water across the basin results in highly variable ANPP estimates


• Preferred dominant grasses, including dropseeds (Sporobolusspp.), black grama, and threeawns (Aristidaspp.) are associated with sandy and gravelly soils, resulting in a tendency for livestock to aggregate in these areas. Heavy use of such areas, especially during drought, leads to rapid and persistent grass loss (chapter 13)


• High Soil Heterogeneity Governs Basin-Level Variation in Key Processes 17


• variation in the depth and development of calcium carbonate - rich soil horizons exerts a strong influence on soils and vegetation. In other areas, the presence of clay-rich horizons can have important positive effects on grass persistence (Gibbens and Beck 1987), and the development of these horizons may be reduced in the presence of high amounts of calcium carbonate in parent materials (Gile et al. 1981).


• The shift from rhyolite and monzonite to limestone derived parent materials across the southern portion of the basin yields shifts in the availability of calcium carbonate as well as rates of weathering, and this affects the composition of plants, the identity of encroaching shrub species, and grass-shrub-animal interactions.


• Biodiversity Exhibits Both Vulnerability and Resilience in a Dynamic Landscape 17


• Some grassland birds present at the time of European colonization may have already been driven regionally extinct and the fauna generally impoverished (Pidgeon et al. 2001). For taxa such as ants, however, the Jornada landscape may be more diverse than desert grasslands with few shrubs due to the abundance of native (and even rare) shrub-associated species (Bestelmeyer et al. 2005). Thus, shrub invasion may have enhanced certain aspects of animal species diversity.


• not enough data on dispersal or patch occupancy for any species to evaluate the potential for habitat fragmentation to reduce species diversity. Consequently, it is not clear what kinds of habitat changes (e.g., exurban development, degrees of shrub encroachment) would produce habitat fragmentation for particular species.


• we recognize that this aggregate reductionist view continues to be constrained by (1) the small-scale, nonhierarchical, and spatially inexplicit nature of many observations and experiments; and (2) the opportunistic (and unfulfilled) integration of results across ecosystem components and individual investigators.


• the value of correlations between local vegetation and local soil properties is limited. For example, satellite imagery and geomorphic studies reveal that the dynamics of northern basin floor positions are governed by eolian fluxes of soil from the border of the Rio Grande Valley, but these fluxes are buffered to the south by the Dona Ana Mountains such that dominant structuring processes become more localized.


• Historical events, including the effects of prehistoric Native American settlements and ranching enterprises of the eighteenth and nineteenth centuries, may have altered the long-term trajectory of localities by initiating desertification processes that continue to unfold. These effects are often unrecognized (and unrecognizable).


• Jornada Basin vegetation is clearly not in equilibrium at any scale, and its changing patterns are a product of both historical events and ongoing processes.


• Generalizations about Arid Rangeland Behavior Are Inherently Limited 17


• regional differences in climate, spatially dominant soils, and the traits of plant species contained within


• Generic Pattern 1: Vegetation on Sandy Soils Is More Resilient than on Clayey Soils 17


• sandy loam and loamy sand soils of the Jornada were dominated by grasses in recent history and most have been converted to eroding shrubland, whereas clay loam soils often continue to be dominated by the original, dominant grasses (Gibbens and Beck 1988).


• Generic Pattern 2: Climatic Variation and Disturbance Reverse Grassland-Shrubland Transitions 17


• drought-induced or age-related mortality of adults is rarely observed (Goslee et al. 2003). [of mesquite/shrubs]


• mesquite may live at least 60 years [same life cycle as termite mounds and the trees that grow in them] on the Jornada (Goslee et al. 2003) and up to 200 years elsewhere (McClaran 2003).


• other shrubs (burroweed, Isocoma tenuisecta)


• Pattern 3: Over Sufficiently Long Time Scales There Is One Domain of Attraction 17


• Grassland-shrubland transitions may appear to involve thresholds separating two domains of attraction over shorter time scales, but a single domain of attraction toward savanna may be revealed over sufficiently long periods, e.g., 40-50 years (Walker 2002; Valone et al. 2002). This is likely to be true in certain cases, even within some Chihuahuan Desert grasslands, but there is no evidence that this is universally true. Some grass-shrub transitions have lasted for at least a century and current processes indicate many will last much longer.


• Resilience times within a domain and the existence of alternative domains are highly variable across the Southwestern United States and within particular landscapes (Bestelmeyer et al. 2003a).


• There are three cornerstone ideas that have underlain rangeland management decisions over the past century. First, it was implicitly assumed that Chihuahuan Desert grasslands possessed a level of resilience to grazing pressure similar to that of other grasslands in North America. This notion led to early management practices based on the notion that the decline of grass abundance following temporary overgrazing could be reversed during periods of increased rainfall. Although domestic livestock grazing had been present in the Jornada Basin since the late 1500s, the emergence of ranching as a commercial enterprise did not take hold until the latter part of the nineteenth century (chapter 13). The practitioners of this new culture immigrated to the Jornada Basin from the mesic prairies to the east and brought their concepts of grassland ecosystem behavior with them. The next century of grazing management practice, policy, and research were affected by those concepts.


• Second, many believed that the primary challenge facing ranchers, researchers, and policy makers was to establish a grazing capacity (maximum stocking rate [ha/animal/year] possible, year after year, without reducing forage to vegetation ratios or other resources; Holechek et al. 1998a) (Jardine and Forsling 1922). A conservative stocking rate was viewed as an appropriate strategy for coping with spatial and temporal climatic variability because "attempts to adjust stocking rate to this highly variable basis of forage have had disastrous results. A breeding herd built up to use most of the forage crop in good or even average years cannot be maintained in dry years" (JER field-day report 1948 unpublished). Using a conservative strategy, adequate forage would be available in most (but not all) years. It was assumed that ungrazed forage produced during favorable years would be available to protect soil or be used as a forage reserve in drought years. It was also implicitly assumed that the infrequent periods of overuse would not have long-term consequences.


• Third, many assumed that a more equitable spatial distribution of livestock grazing pressure would reduce instances of overuse of forage where animals had previously concentrated and underuse in areas that animals had avoided (Jardine and Forsling 1922). Thus, new parts of the landscape were made available for grazing, and provisions of nutrients and fencing have been used to distribute livestock more evenly across the forage resource, presumably reducing impacts on any given point. More recently, Herbel and Nelson (1969) advocated the opportunistic rotation of livestock among pastures to take advantage of spatially variable rainfall, plant production, and plant phenological stage, for example, flowering of soapweed (Yucca elata). Although these strategies accounted for spatial and temporal variation in the vulnerability of forage plants, they did not account for the fine scale of this variation. Most reasonably sized management units encompass significant spatial variability in soil properties, soil resource levels, and vegetation at the patch or patch-mosaic scale. Typical livestock grazing behavior (as currently managed) results in full utilization of palatable forage in patches before moving to the next forage patch (Bailey et al. 1996; Fuhlendorf and Smeins 1997). Therefore, the livestock use in any forage patch is largely inelastic to stocking rate and improved animal distribution.


• most grazing systems employed then, as today, have stocking rates based on a relatively fixed grazing capacity.


• moderate droughts would have required an unrealistic level of economic flexibility on the part of individual ranchers. This suggests that ranchers could not have heeded Herbel and Nelson’s advice in the late 1800s unless they changed their basic operations and their principle reliance on a cow-calf production system. Even today, creative management alternatives such as light stocking rates, fall calving season, and integration of complementary enterprises are strongly encouraged to sustain ranching in the Southwest (Ruyle et al. 2000)


• Recent advances in technologies for tightly controlling animal distribution without fencing (Anderson 2001; Provenza 2003) offer hope that some economic constraints to sustainable grazing can be overcome.


• Although technology offers improved tools for management, ecological solutions that are not tied to socioeconomic innovations are unlikely to stem the tide of grass loss.


• This should not imply that all remaining grasslands can be preserved even if such approaches are successful. Current applications of chemical, mechanical, and management technologies to interdict degrading processes offer little chance of success when the mechanisms driving degradation derive from regional and landscape scales (chapters 14 and 18)


• Our understanding of soil-plant feedback processes and multiscale redistribution of soil resources (chapter 5) implies that simply treating one symptom (e.g., shrub increase) does little to mitigate grassland loss, especially in the short term, over which most economic analysis is performed.


• A basic understanding of ecological processes is a prerequisite for reasonable decisions by land users.


• Two of the conclusions offered by John Wesley Powell in "Report on the Lands of the Arid Region" delivered to Congress in 1878 were that Western lands have distinct limits set by their aridity and cannot be appropriately managed if arbitrarily dissected into fractions by the political conventions of the day (de Buys 2001). In one sense, the history of research in the Jornada Basin has reaffirmed and refined these conclusions.


• Our resource management institutions and principles recognize the necessity of working within biological limits, but social, economic, and administrative constraints often prevent actions based on this knowledge (Ruyle et al. 2000).


• In a socioeconomic setting where property rights are paramount, management of Western lands by fractions has persisted since the nineteenth century, despite Powell's recommendation, with widely reported consequences for biodiversity and human welfare. Lately, however, creative alternatives have emerged around the Western United States that allow resource management to be coordinated over ecologically appropriate regional scales while accommodating ownership of fractions. Examples include grass banks and land management cooperatives.


• In New Mexico, as in much of the Intermountain West, there is a shift from traditional agriculture toward an economy based on services and professional industries (Rasker et al. 2003). For most communities in these regions, future growth will be tightly linked to environmental quality, an amenity often used by industry to attract employees. Nonetheless, livestock grazing continues to be a dominant land use in these regions. The increasing diversity of land uses imposes new values and criteria with respect to the acceptable structure and composition of ecosystems. New land uses also introduce novel processes to particular areas, such as the introduction of nonnative species and habitat fragmentation by roads and houses that increase the demand for information (e.g., the behavior of animal species; Maestas et al. 2003) that has not been a focus of past Jornada Basin research.


• Conclusions 17


• This work has been instrumental in directing desertification research across the globe. Perhaps even more important, the long-term multidisciplinary approach has described remarkable variability of desert ecosystem function, and its causes, across space and time. This perspective reinforces the need to develop scientific and management concepts and methods that account for the unexpected magnitude of ecological variability (Shrader-Frechette and McCoy 1993; O'Neill 2001; Archer and Bowman 2002; Simberloff 2004).


• must adopt truly interdisciplinary approaches in addition to multidisciplinary approaches that were formerly emphasized. We now have the tools to realize the convergence of ecosystem ecology, research, and landscape ecology foreseen by Eugene Odum 40 years ago.


• 18 Future Directions in Jornada Research: Applying an Interactive Landscape Model to Solve Problems


• Agricultural Research Service [ARS] since 1912, New Mexico State University in the late 1920s, and joined by the Long-Term Ecological Research [LTER] program in 1981)


• gaps in our knowledge still remain


• Many studies have documented variable rates and patterns of shrub invasion at the Jornada as well as at other semiarid and arid regions of the world, including the Western United States, northern Mexico, southern Africa, South America, New Zealand, Australia, and China (York and Dick-Peddie 1969; Grover and Musick 1990; McPherson 1997; Scholes and Archer 1997; see also chapter 10)


• In some cases, shrub invasion occurred very rapidly: At the Jornada, areas dominated by perennial grasses decreased from 25% to #7% from 1915 to 1998 with most of this conversion occurring prior to 1950 (Gibbens et al. 2005; Yao et al. 2002a).


• In other cases, shrub invasion occurred slowly


• Soil texture, grazing history, and precipitation patterns are insufficient to account for this variation in grass persistence through time (Yao et al. 2002a)


• although many attempts to remediate these shrublands back to perennial grasses have led to failure, some methods worked well, albeit with long ( greater than 50 year) time lags (Rango et al. 2002; see also chapter 14).


• In arid systems, aboveground net primary production (ANPP) can vary three- to fivefold, both between years at the same location and within the same year at different locations (Huenneke et al. 2001, 2002; see also chapter 11)


• Given the importance of arid and semiarid systems to current global issues, including carbon dynamics (Houghton et al. 1999; Pacala et al. 2001; Jackson et al. 2002), loss of biodiversity (Whitford et al. 2001), invasion of exotic species (DiTomaso 2000; Masters and Sheley 2001), wind and water erosion of soil and nutrients (Schlesinger et al. 1999; Wainwright et al. 2000), and emission of dust loads to the atmosphere (Gillette et al. 1992; Tegen et al. 1996; Gillette and Chen 2001), there is a critical need for a better understanding of ecosystem patterns and dynamics at multiple spatial and temporal scales.


• focuses on three interrelated aspects of landscapes: (1) feedbacks among plants, animals, and soils generated from interactions among biotic processes, a heterogeneous physical template, and the disturbance regime across a range of spatial and temporal scales; (2) neighborhood or contagious processes that generate fluxes and flows within and among spatial units; and (3) landscape characteristics, including the structure and spatial distribution of spatial units as well as the landscape context or the condition of the study area of interest relative to its surroundings that modify the transfers of materials.


• general consensus does not exist regarding the key factors controlling different outcomes of shrub invasion under similar conditions.


• Periodic droughts on 20-60-year cycles have variable effects


• Precipitation also has high variability between years


• Soil properties are also spatially and temporally heterogeneous - further modified by extensive redistribution by both wind and water (Gile et al. 1981; McAuliffe 1994; Wondzell and Ludwig 1995).


• Steep slopes exacerbate the effect of high-intensity convective storms (Wondzell et al. 1996). Even subtle differences in elevation (#0.6# slope) combined with plant-soil water feedbacks can generate "striped" patterns in vegetation (Montan a et al. 1990; Aguiar and Sala 1999).


• Lightning ignited fires occur primarily in grasslands where fuel loads are sufficiently high and spatially continuous to carry a fire. Historically, fire may have played an important role in limiting shrub invasion, but the current dominance by shrubs throughout many arid landscapes limits the ability of fires to spread (Drewa et al. 2001).


• well-known resource-redistribution model of Schlesinger et al. (1990) [general patches affected by rain/graizing, islands of fertility, wind/water erosion, ]


• [new models] includes (1) variation in biotic and abiotic processes and the disturbance regime as well as feedbacks among these system components, (2) landscape characteristics, and (3) contagious or neighborhood processes that connect different plants, patches, and landscape units (figure 18-1).


• patches vary in size from several individual plants (less than 5m2 ) to several hundred individuals (greater than 1,000 m2)


• The configuration of patches (i.e., size, number, and adjacency or between-plant distance) can have important effects on patch dynamics as well as on the function of the landscape unit. Important landscape units at the Jornada are bajadas, sandy basins, and playas.


• The dynamics of each spatial unit are determined by local physical and biotic factors within the unit and transfers of materials among units (figures 18-2 and 18-3). Local physical factors that exert major controls on semiarid and arid ecosystem dynamics include geomorphology and soils, precipitation, temperature, and disturbance history. Geomorphic constraints include parent material, elevation, and slope aspect, length, and steepness that control the location of runoff and run-in areas (Monger 1999; see also chapters 4 and 7). Soil characteristics determine (1) the capture, retention, and supply of water; (2) the supply of nutrients through mineral weathering and organic matter mineralization; (3) erosion rates; and (4) the environment for root growth and soil biotic activity. Local precipitation inputs and temperature can vary among landscape units due to differences in elevation and location relative to areas with high topographic relief. Disturbance history is particularly important in areas such as the Jornada, where active management has occurred for decades.


• An increase in summer precipitation may promote the establishment of short-lived herbaceous species on some shrub-dominated sites (Peters and Herrick 1999b). This increase in plant biomass will result in an increase in soil organic matter and infiltration capacity of the soil with feedbacks to plant-available water and perennial grass establishment and recovery following shrub invasion.


• Large herbivores respond to individual plants, patches, and landscape units that are often nonuniformly distributed across the landscape (Senft et al. 1987; Bailey et al. 1996). The response of large animals depends on the distribution of these spatial units of forage quality and quantity as well as abiotic features, such as topography and distance to water.


• the expansion of native shrubs into perennial grasslands occurs as a result of cattle (Bos) consuming seeds from species such as honey mesquite that have palatable pods. Viable seeds that pass through the animal's digestive tract can be redistributed at large distances from the source plant population.


• a high density of bannertail kangaroo rats (Dipodomys spectabilis) in grasslands versus a greater abundance of jackrabbits (Lepus) and rodents following shrub invasion (Moroka et al. 1982; Whitford 1997) has important effects on patterns and intensity of herbivory, granivory, and seed dispersal (chapter 12).


• Transfers of material can occur within as well as among spatial units (patch to patch) and between hierarchical levels (plant to patch and vice versa) (figure 18-3).


• The major vectors of redistribution are water, wind, small and large animals, and disturbances such as fire and human activities (figure 18-3)


• Water is also an important dispersal agent for seeds, in particular for large-seeded species, such as creosotebush and sideoats grama (Bouteloua curtipendula) (Gutierrez-Luna 2000).


• [soils] protected [from wind erosion] by vegetation or a strong physical or biological crust.


• [cows] deposit viable seeds in feces at large distances (several km) from the seed source (Janzen 1982; Chambers and MacMahon 1994).


• [fire] responds to vegetation fuel load and to environmental conditions of wind speed, temperature, and humidity


• By reducing plant cover, fire increases the redistribution of soil particles through wind and water erosion (Johansen et al. 2001; Whicker et al. 2002). Fire also releases particles into the atmosphere that can affect regional air quality (Wotawa and Trainer 2000).


• human activities alter vegetative cover and biodiversity with increasing fragmentation of the landscape


• Dust loads in the atmosphere also increase during dry, windy periods with increasing urbanization and clearing of marginal land for industry and agriculture; this sequence of events occurred following the 1930s drought and has been cited as the primary cause of increases in suspended particulate matter in urban areas in the Southwestern United States.


• The relative importance of transfers of materials within and among spatial scales is affected by (1) landscape characteristics, including structure (size, shape, and type of spatial units); (2) spatial distribution or configuration of spatial units; and (3) the context or location and characteristics of an area relative to its surroundings or nearby areas.


• These characteristics influence the connectivity of the landscape by modifying the ability of vectors of redistribution (water, wind, animals, and disturbance) to move materials horizontally. Highly connected landscapes consist of a mosaic of spatial units distributed in such a way as to promote the movement of materials via spatial processes, whereas landscapes with low connectivity may have barriers or spatial configurations of units that restrict horizontal movement of materials.


• at the Jornada, a 250-ha cattle exclosure was constructed in 1933 such that the northwest part of the exclosure was dominated by grassland and the southern part was dominated by honey mesquite (figure 18-4). Although cattle were excluded from the exclosure, mesquite continued to recruit and expand in the surrounding pasture that was grazed as well as in the exclosure as a result of the dispersal of mesquite seeds from small animals. Local dispersal from mesquite plants within the exclosure at the time of construction was also likely. From 1948 to 1987 the grazed pasture continued to fill in with new mesquite plants, and the established plants grew larger. At some point between 1987 and 1998, the density and spatial configuration of mesquite plants crossed a critical threshold such that wind erosion became prevalent and coppice dunes developed throughout both the grazed pasture and the nearby exclosure. These legacies, such as the historic location of shrub communities, can be particularly important in understanding current patterns in vegetation and soils.


• Few studies have included landscape context when examining factors influencing shrub invasion and grass persistence. Most studies have examined plot-level characteristics and have not accounted for the influence of the surrounding area.


• Analysis showed that distance to shrubland in 1915 was the best predictor of grass persistence in 1999; the farther a plot was from an existing shrubland, the greater the probability that grasses would persist through time (Yao et al. 2002a)


• Other explanatory factors commonly used in shrub invasion studies, such as soil texture, grazing intensity, and precipitation, were less important than distance to the nearest shrub-dominated areas.


• combine remotely sensed images with field data and spatial databases


• three pressing ecological issues: shrub invasion, remediation, and carbon storage and dynamics


• Grasslands occurred primarily either on level uplands with sandy soils dominated by black grama or on playas or basin floors that received run-in water and were dominated by tobosa grass and burrograss. Shrublands occurred either on sites with shallow, calcareous soils susceptible to water erosion that were dominated by creosotebush (e.g., many upper alluvial fans) or on deep, sandy soils located along stream channels and arroyos that were dominated by honey mesquite.


• two most common shrub species in these systems: honey mesquite and creosotebush.


• [cattle both distribute mesquite seeds, but help them get established by grazing the surrounding grass]


• Creosotebush seeds are relatively large and unpalatable, thus dispersal was most likely through sheet flow of water or in stream channels during floods. Constraints on seed germination and seedling establishment of creosotebush are poorly understood in the Chihuahuan Desert. observations suggest that recruitment events are episodic with largescale events occurring following the droughts in the late 1800s and again in the 1950s. Recruitment at this time was likely promoted by the reduced grass cover due to heavy grazing by cattle over this time period (Fredrickson et al. 1998).


• Remediation 18


• successes to indicate that the system can be manipulated (e.g., Cassady and Glendening 1940)


• Most remediation attempts in arid and semiarid ecosystems have had limited success for three major reasons: (1) the key processes limiting vegetation recovery at different spatial and temporal scales have not been identified, (2) nonlinear thresholds related to vegetation-soil-animal interactions and feedbacks have been crossed, and (3) the landscape context and importance of linkages among spatial units within a landscape have been ignored (Archer 1989; Peters and Betancourt 2001).


• take advantage of extreme events, such as El Nino and drought


• Prioritizing efforts by concentrating on locations likely to change under certain conditions and focusing on key processes will provide guidelines and recommendations for future remediation efforts (Herrick et al. 1997).


• Recent estimates of carbon sinks in the coterminous United States from 1980 to 1990 indicate that grasslands and shrublands may account for similarly large amounts of carbon storage as in forests (Pacala et al. 2001).


• The redistribution of carbon and soil nutrients across a landscape may be as important to vegetation dynamics as local inputs, especially for nitrogen (chapter 6)


• Most estimates for carbon sinks and losses have a high degree of uncertainty due to landscape-scale variation in edaphic and topographic factors (Schlesinger and Pilmanis 1998; Pacala et al. 2001; Hurtt et al. 2002). Providing estimates of carbon dynamics based on an average value for a landscape may be misleading. Some vegetation types will have extremely high standing biomass and production, whereas other types will have very low values (Huenneke et al. 2002).


• High spatial and temporal variation in ecosystem dynamics across multiple scales cannot be explained using current models of these systems (e.g., Schlesinger et al. 1990). We developed an interactive landscape model that incorporates three key properties of landscapes: (1) feedbacks among plants, animals, and soils; (2) contagious processes that generate fluxes and flows within and among spatial units; and (3) landscape characteristics, including structure and spatial configuration of spatial units as well as the characteristics of the study area or its context relative to its surroundings.